Retire the SQLite backend — Postgres is the only engine

The dual backend (SQLite via db.js + Postgres via schema.pg.sql) was a
maintenance foot-gun: a schema change could land on the SQLite path only and
silently 500 every read on prod (it just did, with #18/#13). Production has run
on Postgres for weeks, so SQLite is retired: ONE schema source of truth
(db/schema.pg.sql), no drift possible.

- dbx.js: default DB_BACKEND=pg; an unknown backend now fails loudly at require
  time instead of silently selecting a stale engine.
- Deleted server/db.js, server/db/sqlite.js, server/db/migrate-sqlite-to-pg.js,
  server/scripts/migrate-bizgaze-only.js (all SQLite-only, none in the runtime
  path — the running server loads db/pg.js).
- Tests (e2e, db-smoke) target Postgres now and fail-fast (skip) unless
  DATABASE_URL points at a disposable test DB — never SQLite, never prod.
- Removed the dead DB_PATH env + fixed misleading SQLite comments in the
  Dockerfile / docker-compose (kept the /data volume: it holds
  uploads/recordings/transcripts/downloads, not just the old data.db).
- CLAUDE.md: stack + repo-layout + run-locally updated for Postgres-only.

Runtime is unaffected (prod already sets DB_BACKEND=pg and pg is a prod dep);
this only removes the unused SQLite path.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker-compose.yml

@@ -10,7 +10,6 @@ services:
     restart: unless-stopped
     environment:
       - PORT=8090
-      - DB_PATH=/data/data.db
       # Desktop installers + auto-update feed live on the persistent volume so uploaded
       # builds survive image rebuilds (a plain image path would be wiped on every deploy).
       - DOWNLOADS_DIR=/data/downloads
@@ -31,16 +30,16 @@ services:
       - path: .env
         required: false
     volumes:
-      - bizgaze_support_data:/data   # persists data.db across rebuilds
+      - bizgaze_support_data:/data   # persists uploads / recordings / transcripts / downloads across rebuilds
     networks:
       - npm
-    # Wait for Postgres to be healthy before starting. Only matters when DB_BACKEND=pg (else the app uses
-    # the local SQLite file and ignores pg), but it's harmless on SQLite — pg comes up in a second or two.
+    # The DB is Postgres (SQLite retired 2026-08-12), so wait for it to be healthy before the app starts —
+    # otherwise the first queries race the DB coming up.
     depends_on:
       bizgazepg:
         condition: service_healthy
 
-  # PostgreSQL — the app's data store when DB_BACKEND=pg (default is the local SQLite file). Distinct
+  # PostgreSQL — the app's data store (DB_BACKEND=pg; the only backend since SQLite was retired). Distinct
   # service/container name so it never collides with the other postgres containers on the shared NPM
   # network; the app reaches it as `bizgaze-postgres`. Data on its own named volume.
   bizgazepg: