feat(rw-comments): process doc comments via a backend extension point

[yumike]

What

Replaces the rw.comments events bus with a backend extension point, decoupling the doc-comment notification mechanism from the events service.

• New package @rwdocs/backstage-plugin-rw-node — pure types + rwCommentProcessingExtensionPoint (addProcessor) + CommentProcessor / CommentActivity. Only dependency @backstage/backend-plugin-api; no @rwdocs/core, no knex.
• rw-backend — CommentActivityResolver resolves a self-contained CommentActivity from its own tables (plus a catalog lookup for the resolved actor name); a fire-and-forget CommentPostProcessor fans it out to every registered processor and is injected into the comments router. The events bus + CommentEventPublisher + rw-common's commentEvents.ts are removed.
• notifications module — CommentNotifier implements CommentProcessor (format + send only) and registers via addProcessor. No DB, events, or catalog.

All resolver reads are best-effort and run concurrently, so one transient failure can't reject the resolve and suppress an otherwise-deliverable notification.

Frozen contract (preserved byte-for-byte)

• topics comment:thread:created / comment:reply:created / comment:thread:resolved
• scope rw:comment:${rootId}
• title/description/link wording incl. the "the docs" and "Someone" fallbacks

Verification

End-to-end in the browser (Alice/Bob, yarn dev): all three topics, actor exclusion on both paths, and per-thread scope collapse confirmed. typecheck 0, 238 rw-backend tests pass, lint + format clean.

🤖 Generated with Claude Code