fix: migrate E2E tests from SQLite to Postgres

- Rewrite e2e/seed.ts to use postgres driver instead of bun:sqlite
- Add userId to all seeded entities (multi-user schema)
- Add Postgres service container to CI E2E job
- Remove DATABASE_PATH from test server start script
- Re-enable E2E job in CI

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

e2e/start-test-server.sh

@@ -1,4 +1,4 @@
 #!/bin/sh
 # Seed the test database, then start the production server
 bun run e2e/global-setup.ts
-NODE_ENV=production DATABASE_PATH=./e2e/test.db bun run src/server/index.ts
+NODE_ENV=production bun run src/server/index.ts