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>

server/test/e2e.js

@@ -5,12 +5,14 @@
 // (Login currently marks the session MFA-passed directly, so there is no separate
 // TOTP step in the product flow; the MFA endpoints still exist but aren't exercised here.)
 
-const fs = require('fs');
-const os = require('os');
-const path = require('path');
-const DB = path.join(os.tmpdir(), 'ra-e2e.db');
-process.env.DB_PATH = DB;
-for (const f of [DB, DB + '-wal', DB + '-shm']) { try { fs.unlinkSync(f); } catch {} }
+// SQLite was retired 2026-08-12 — the backend is Postgres now. Point DATABASE_URL at a DISPOSABLE test DB
+// (never production — this creates + mutates rows), e.g.:
+//   DATABASE_URL=postgres://bizgaze:PW@localhost:5432/bizgaze_test node test/e2e.js
+process.env.DB_BACKEND = process.env.DB_BACKEND || 'pg';
+if (!process.env.DATABASE_URL) {
+  console.log('SKIP e2e: set DATABASE_URL to a disposable Postgres test DB (SQLite retired 2026-08-12).');
+  process.exit(0);
+}
 
 const PORT = 8099;
 process.env.PORT = PORT;