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/db/pg.js

@@ -1,6 +1,6 @@
-// PostgreSQL backend for the async DB adapter. Same interface as db/sqlite.js — prepare(sql).{get,all,run},
-// exec(sql), tx(fn), init() — so repos and app code are engine-agnostic. Selected by DB_BACKEND=pg;
-// connection string from DATABASE_URL.
+// PostgreSQL backend for the async DB adapter — the ONLY backend (SQLite retired 2026-08-12). Implements
+// prepare(sql).{get,all,run}, exec(sql), tx(fn), init() so repos/app code stay engine-agnostic (the facade
+// in dbx.js keeps the door open for future backends). Connection string from DATABASE_URL.
 const { Pool, types } = require('pg');
 const fs = require('fs');
 const path = require('path');