ci: add GitHub Actions workflow (staged in ci/ pending workflow-scope push)

Commit notes

Adds a ready-to-use CI workflow. It is placed under ci/ rather than .github/workflows/ because this environment's OAuth app lacks the 'workflow' scope required to push workflow files (and the GitHub contents API rejected the write). See ci/README.md for the one-step activation (git mv into .github/workflows/).

The workflow's test job runs the backend suite against pgvector Postgres + Redis service containers, activating the DB-backed integration suites (auth.routes, purchases.integration) that skip without a database. lint + typecheck run as informational jobs due to pre-existing uploads.ts dead-code failures.