fix(deploy): pass the documented postgres tuning values to postgres

deploy/.env.example documents POSTGRES_MAX_CONNECTIONS,
POSTGRES_SHARED_BUFFERS, POSTGRES_EFFECTIVE_CACHE_SIZE and
POSTGRES_MAINTENANCE_WORK_MEM, with notes on how to size them — but no
compose file ever passes them to the postgres container. A user who sets
them in .env gets nothing, silently.

Wire them into the postgres command in deploy/docker-compose.yml. The
fallbacks are the postgres:18 stock defaults (100 / 128MB / 4GB / 64MB),
so a deploy that does not set the variables behaves exactly as before.

Checked with postgres:18-alpine: with the variables unset, SHOW gives
the stock values; with them set (1024 / 1GB / 6GB / 128MB), SHOW gives
the set values.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

deploy/docker-compose.yml

@@ -202,6 +202,15 @@ services:
       nofile:
         soft: 100000
         hard: 100000
+    # Pass the tuning values documented in .env.example to postgres. The
+    # fallbacks are the postgres:18 stock defaults, so nothing changes when
+    # the variables are not set.
+    command: >
+      postgres
+      -c max_connections=${POSTGRES_MAX_CONNECTIONS:-100}
+      -c shared_buffers=${POSTGRES_SHARED_BUFFERS:-128MB}
+      -c effective_cache_size=${POSTGRES_EFFECTIVE_CACHE_SIZE:-4GB}
+      -c maintenance_work_mem=${POSTGRES_MAINTENANCE_WORK_MEM:-64MB}
     volumes:
       - postgres_data:/var/lib/postgresql/data
     environment: