fix: install the DI middleware on every broker, on startup

[lesnik512]

Closes #42

What

setup_di installed the DI middleware on app.broker only. FastStream 0.7 apps hold a list of brokers (FastStream(*brokers), app.add_broker), and app.broker is just brokers[0], so every other broker ran without DI. The failure was silent at setup: the first message to a FromDI subscriber on another broker died with AttributeError: 'NoneType' object has no attribute 'resolve_dependency', because ContextRepo.get returns None for the never-set request container key.

How

setup_di no longer calls add_middleware itself. It registers an on_startup hook that walks app.brokers and adds the middleware factory to each broker that does not already carry it.

• Startup is the one moment when the broker list is complete and nothing has been consumed, so a broker added with app.add_broker after setup_di is covered without documenting an ordering rule (the issue's options 1 and 2 both miss that case; option 3 refuses a shape FastStream supports).
• It is safe because FastStream builds a subscriber's middleware stack per message from config.broker_middlewares, so a middleware added in a startup hook applies to subscribers registered earlier.
• The membership check reads broker.config.broker_middlewares, the same sequence FastStream builds the stack from, so a stop/start cycle does not add a second copy.
• The if not app.broker guard is unchanged. Relaxing it for brokers created inside a user's own on_startup hook is possible but depends on hook ordering, so it is left for its own issue.

Recorded in docs/adr/0002-install-middleware-on-startup.md, including the revisit trigger.

Tests

• test_di_resolves_on_every_broker, parametrized over both brokers at construction and the second broker added after setup_di. Both cases failed on main.
• test_middleware_is_installed_once_per_broker_across_restarts guards the idempotency check across two TestApp cycles.
• just lint-ci clean, just test-ci at 100% coverage.

Follow-ups not in this PR

• Raise a RuntimeError naming setup_di from Dependency.__call__ when the request container is missing, instead of the current AttributeError on None.
• Relax the no-broker guard, see above.