Example gallery: full runnable reference applications (CI-tested)

## Motivation

The existing quickstarts are CI-tested and good; what's missing is the next step — full reference applications showing Awa embedded in the frameworks people actually run, exercising the integration seams (bridges, callbacks, mixed fleets) that quickstarts can't.

## Scope

CI-tested reference apps under `examples/`, one directory each:

- **FastAPI + Awa** — transactional enqueue via `awa.bridge.insert_job`, a user-owned callback endpoint (#293 pattern), progress polling.
- **Django (sync)** — `insert_job_sync` inside a request transaction, `Client` sync worker.
- **axum + SeaORM** — `awa-seaorm` transactional enqueue alongside application writes.
- **Serverless tick()** — the #118 shape driven by a cron trigger, HttpWorker execution.
- **Mixed pipeline** — Rust producer enqueuing for a Python ML worker on the same queue, with a follow-up job (ADR-029) and an A→B dependency once #14 lands.

Each app: docker-compose for Postgres, a README that doubles as a tutorial, and a smoke test in CI (same convention as the quickstarts).

## Acceptance

- All apps green in CI; each README walks from clone to observed job completion.
- Linked from the docs-site Learn section and the relevant feature docs.

## References

Roadmap: `docs/0.7-roadmap.md` (WS-6, NEW-16). Related: #118, #14, #293, ADR-029, `awa-seaorm`.

Start with docs/0.7-roadmap.md and the existing CI-tested quickstarts to understand the WS-6/NEW-16 scope and conventions. Add the five reference-app directories under examples/, each with Docker Compose, a tutorial README, and a CI smoke test; done means every app reaches observed job completion in CI and is linked from the docs-site Learn section and relevant feature docs.

Written by the indexing model from the issue text.