Release 0.5.0: PR-less per-RFC discussion (contribution still requires PR)

Roadmap item #3. An RFC's main view now carries a discussion surface
distinct from PR comments and from branch chat. The substrate is the
existing threads/thread_messages tables — rows with branch_name IS NULL
scope to the RFC's main view; the schema already permitted that shape,
v0.5.0 is the first build to write it. Five new endpoints under
/api/rfcs/<slug>/discussion/..., a new RFCDiscussionPanel right-column
component used when branchParam === main, SPEC §10.10 settling
discussion-vs-contribution, and §17 listing the new routes. Notification
routing reuses the existing chat_message_in_participated_thread /
chat_reply_to_my_message event kinds with branch_name=null on the
fan-out row; a distinct event_kind is a §19.2 candidate. Anonymous
viewers can read; writes require contributor — v0.6.0's item #4 will
harden adjacent gates. No schema migration; minor bump, no operator
action required beyond rebuild and restart.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/api.py

@@ -20,6 +20,7 @@ from pydantic import BaseModel, Field
 from . import (
     api_admin,
     api_branches,
+    api_discussion,
     api_graduation,
     api_notifications,
     api_prs,
@@ -81,6 +82,12 @@ def make_router(
     # the §15.8 mute typeahead) and the §6/§17 admin surfaces
     # (role, write-mute, audit-log, graduation-readiness queue).
     router.include_router(api_admin.make_router(config))
+    # v0.5.0: §5 / §7 / §10 — PR-less per-RFC discussion endpoints.
+    # The substrate is the existing threads/thread_messages tables;
+    # rows whose branch_name IS NULL scope to the RFC's main view.
+    # Contribution still requires a PR (api_prs above); this surface
+    # is for discussion that does not yet warrant a branch.
+    router.include_router(api_discussion.make_router())
 
     # ---------------------------------------------------------------
     # §17: /api/health — unauthenticated post-flight probe.