feat(broadcasting): add channels.py publishable stub for Reverb

[bedus-creation]

Summary

• Adds fastapi_startkit/src/fastapi_startkit/broadcasting/stubs/channels.py — the publishable stub that ReverbProvider will copy to routes/channels.py when a developer runs the publish command
• The stub includes a fully-documented @Broadcast.channel("orders.{order_id}") example with typed wildcard injection so developers know the authorization pattern immediately
• Adds the stubs/__init__.py package marker

Related

• Part of tasks #150 and #151 (Reverb template docs)
• application/routes/channels.py added in the fastapi-startkit application template repo
• docs/reverb.md added in the fastapi-startkit.github.io docs repo

Test plan

• Verify stubs/channels.py is importable
• Confirm the stub content matches the @Broadcast.channel pattern documented in docs/reverb.md

🤖 Generated with Claude Code

[bedus-creation]

Review — Superseded by PR #112

This PR adds fastapi_startkit/src/fastapi_startkit/broadcasting/stubs/channels.py (and the stubs/__init__.py), which is the same file added by the canonical PR #112 (feat/broadcasting-event-facade-provider).

PR #112 is the authoritative implementation for tasks #147–#150. It already includes:

• broadcasting/stubs/__init__.py
• broadcasting/stubs/channels.py (with Broadcast facade examples and commented scaffold)

Recommendation: Close this PR as superseded by #112. Merging both would cause a conflict on the stub files, and the content in #112 already covers this use case.

Once #112's two blocking issues are fixed and it is merged, the stub will be available via artisan publish.

[bedus-creation]

Superseded — channels.py stub already shipped in merged PR #112.