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>

Dockerfile

@@ -1,5 +1,6 @@
 # BizGaze Support — server image
-# Node 24 ships node:sqlite as a stable built-in (no flag), which db.js relies on.
+# Data store is PostgreSQL (DB_BACKEND=pg; the `pg` client is a prod dependency installed below). SQLite was
+# retired 2026-08-12 — one schema source of truth (db/schema.pg.sql), no dual-maintenance drift.
 FROM node:24-alpine
 
 # ffmpeg: server-side video poster thumbnails (see static.js /thumbs/<id>). Small on Alpine.
@@ -15,9 +16,9 @@ RUN npm install --omit=dev --no-audit --no-fund
 # App source
 COPY server/ ./
 
-# Served HTTP port (NPM terminates TLS and proxies here). DB lives on a volume.
+# Served HTTP port (NPM terminates TLS and proxies here). The DB is Postgres (DATABASE_URL, set via .env);
+# the /data volume holds uploads / recordings / transcripts / downloads (see docker-compose.yml).
 ENV PORT=8090
-ENV DB_PATH=/data/data.db
 EXPOSE 8090
 
 CMD ["node", "server.js"]