fix(hub): reindex usage events to backfill model attribution

#1359's session-model fallback only applies to events indexed after
the upgrade. Clear usage_scan_state in a V19→V20 migration so the
lazy re-index re-derives legacy rows instead of leaving them
bucketed as "unknown".

hub/src/store/index.ts

@@ -33,7 +33,7 @@ export { SessionStore } from './sessionStore'
 export { UserStore } from './userStore'
 export { UsageStore } from './usageStore'
 
-const SCHEMA_VERSION: number = 19
+const SCHEMA_VERSION: number = 20
 const REQUIRED_TABLES = [
     'sessions',
     'machines',
@@ -287,6 +287,7 @@ export class Store {
             16: () => this.migrateFromV16ToV17(),
             17: () => this.migrateFromV17ToV18(),
             18: () => this.migrateFromV18ToV19(),
+            19: () => this.migrateFromV19ToV20(),
         })
 
         if (currentVersion === 0) {
@@ -809,6 +810,24 @@ export class Store {
         `)
     }
 
+    private migrateFromV19ToV20(): void {
+        // tiann/hapi#1359 stamps a stable session-model fallback onto usage
+        // events that predate model attribution. That only helps events
+        // indexed after the upgrade, so clear the scan state once to force
+        // usageService's lazy re-index to re-derive every event. The events
+        // table is left alone: the re-index replaces each session's rows and
+        // reuses any previously indexed explicit models.
+        this.db.exec(`
+            CREATE TABLE IF NOT EXISTS usage_scan_state (
+                session_id TEXT PRIMARY KEY,
+                message_epoch INTEGER NOT NULL DEFAULT 0,
+                last_seq INTEGER NOT NULL DEFAULT 0,
+                FOREIGN KEY (session_id) REFERENCES sessions(id) ON DELETE CASCADE
+            );
+            DELETE FROM usage_scan_state;
+        `)
+    }
+
     private getSessionColumnNames(): Set<string> {
         const rows = this.db.prepare('PRAGMA table_info(sessions)').all() as Array<{ name: string }>
         return new Set(rows.map((row) => row.name))

hub/src/store/migration-v13.test.ts

@@ -10,7 +10,7 @@ describe('Store V12/V13→V14 schema reconciliation', () => {
         const store = new Store(':memory:')
         expect(tableExists(store, 'message_epochs')).toBe(true)
         expect(tableExists(store, 'session_scratchlist')).toBe(true)
-        expect(getUserVersion(store)).toBe(19)
+        expect(getUserVersion(store)).toBe(20)
         store.close()
     })
 
@@ -34,7 +34,7 @@ describe('Store V12/V13→V14 schema reconciliation', () => {
             store = new Store(dbPath)
             expect(tableExists(store, 'message_epochs')).toBe(true)
             expect(tableExists(store, 'session_scratchlist')).toBe(true)
-            expect(getUserVersion(store)).toBe(19)
+            expect(getUserVersion(store)).toBe(20)
             expect(store.messages.getMessageEpoch('session-1')).toBe(0)
             expect(store.messages.getMessages('session-1')).toHaveLength(1)
         } finally {
@@ -72,7 +72,7 @@ describe('Store V12/V13→V14 schema reconciliation', () => {
             store = new Store(dbPath)
             expect(tableExists(store, 'message_epochs')).toBe(true)
             expect(tableExists(store, 'session_scratchlist')).toBe(true)
-            expect(getUserVersion(store)).toBe(19)
+            expect(getUserVersion(store)).toBe(20)
             expect(store.messages.getMessages('session-1')).toHaveLength(1)
         } finally {
             store?.close()

hub/src/store/migration-v15.test.ts

@@ -19,7 +19,7 @@ describe('Store V14→V15 migration: scratchlist attachments column', () => {
         expect(cols).toContain('attachments')
         expect(getColumns(store, 'usage_events')).toContain('last_input_tokens')
         expect(getColumns(store, 'usage_scan_state')).toContain('last_seq')
-        expect(getUserVersion(store)).toBe(19)
+        expect(getUserVersion(store)).toBe(20)
         store.close()
     })
 
@@ -40,7 +40,7 @@ describe('Store V14→V15 migration: scratchlist attachments column', () => {
             expect(cols).toContain('attachments')
             expect(getColumns(store, 'usage_events')).toContain('last_input_tokens')
             expect(getColumns(store, 'usage_scan_state')).toContain('last_seq')
-            expect(getUserVersion(store)).toBe(19)
+            expect(getUserVersion(store)).toBe(20)
         } finally {
             store?.close()
             rmSync(dir, { recursive: true, force: true })
@@ -60,7 +60,7 @@ describe('Store V14→V15 migration: scratchlist attachments column', () => {
             store2 = new Store(dbPath)
             const cols2 = getColumns(store2, 'session_scratchlist')
             expect(cols2).toEqual(cols1)
-            expect(getUserVersion(store2)).toBe(19)
+            expect(getUserVersion(store2)).toBe(20)
         } finally {
             store2?.close()
             store1?.close()

hub/src/store/migration-v18.test.ts

@@ -38,7 +38,7 @@ describe('Store V18->V19 migration: usage scan state', () => {
             const usageRows = internalDb.prepare('SELECT COUNT(*) AS count FROM usage_events').get() as { count: number }
 
             expect(table?.name).toBe('usage_scan_state')
-            expect(version.user_version).toBe(19)
+            expect(version.user_version).toBe(20)
             expect(usageRows.count).toBe(0)
         } finally {
             store?.close()

hub/src/store/migration-v19.test.ts

@@ -0,0 +1,101 @@
+import { describe, expect, it } from 'bun:test'
+import { Database } from 'bun:sqlite'
+import { mkdtempSync, rmSync } from 'node:fs'
+import { join } from 'node:path'
+import { tmpdir } from 'node:os'
+import { Store } from './index'
+import { getUsageSummary } from '../sync/usageService'
+
+describe('Store V19->V20 migration: usage re-index for model fallback', () => {
+    it('clears usage scan state while keeping derived events', () => {
+        const directory = mkdtempSync(join(tmpdir(), 'hapi-migration-v19-to-v20-'))
+        const dbPath = join(directory, 'test.db')
+        let store: Store | undefined
+        try {
+            store = new Store(dbPath)
+            store.close()
+            store = undefined
+
+            const db = new Database(dbPath, { create: true, readwrite: true, strict: true })
+            db.exec(`
+                INSERT INTO sessions (id, created_at, updated_at)
+                VALUES ('session-1', 1, 1);
+                INSERT INTO usage_events (
+                    session_id, source_key, source_seq, created_at, agent, kind
+                ) VALUES (
+                    'session-1', 'legacy-key', 1, 1, 'codex', 'cumulative'
+                );
+                INSERT INTO usage_scan_state (session_id, message_epoch, last_seq)
+                VALUES ('session-1', 1, 42);
+                PRAGMA user_version = 19;
+            `)
+            db.close()
+
+            store = new Store(dbPath)
+            const internalDb = (store as unknown as { db: Database }).db
+            const version = internalDb.prepare('PRAGMA user_version').get() as { user_version: number }
+            const scanRows = internalDb.prepare('SELECT COUNT(*) AS count FROM usage_scan_state').get() as { count: number }
+            const usageRows = internalDb.prepare('SELECT COUNT(*) AS count FROM usage_events').get() as { count: number }
+
+            expect(version.user_version).toBe(20)
+            expect(scanRows.count).toBe(0)
+            expect(usageRows.count).toBe(1)
+        } finally {
+            store?.close()
+            rmSync(directory, { recursive: true, force: true })
+        }
+    })
+
+    it('re-indexes legacy unknown-model events with the session model fallback', () => {
+        const directory = mkdtempSync(join(tmpdir(), 'hapi-migration-v19-to-v20-reindex-'))
+        const dbPath = join(directory, 'test.db')
+        let store: Store | undefined
+        try {
+            store = new Store(dbPath)
+            const session = store.sessions.getOrCreateSession(
+                'session-1',
+                { path: '/tmp', host: 'test', flavor: 'codex' },
+                null,
+                'default',
+                'gpt-test'
+            )
+            store.messages.addMessage(session.id, {
+                role: 'agent',
+                content: {
+                    type: 'codex',
+                    data: {
+                        type: 'token_count',
+                        thread_id: 'thread-1',
+                        turn_id: 'turn-1',
+                        info: {
+                            total_token_usage: { input_tokens: 100, output_tokens: 10 },
+                            last_token_usage: { input_tokens: 100, output_tokens: 10 }
+                        }
+                    }
+                }
+            })
+            expect(getUsageSummary(store, 'default', 'all').totals.requests).toBe(1)
+            store.close()
+            store = undefined
+
+            // Simulate the pre-#1359 state: derived rows without model
+            // attribution and a scan state that blocks any re-index.
+            const db = new Database(dbPath, { create: true, readwrite: true, strict: true })
+            db.exec(`
+                UPDATE usage_events SET model = NULL;
+                PRAGMA user_version = 19;
+            `)
+            db.close()
+
+            store = new Store(dbPath)
+            const result = getUsageSummary(store, 'default', 'all')
+            expect(result.totals.requests).toBe(1)
+            expect(result.byModel).toEqual([
+                expect.objectContaining({ key: 'gpt-test', totalTokens: 110 })
+            ])
+        } finally {
+            store?.close()
+            rmSync(directory, { recursive: true, force: true })
+        }
+    })
+})