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/notify.py

@@ -212,7 +212,7 @@ def fan_out_chat_message(
     *,
     actor_user_id: int,
     rfc_slug: str,
-    branch_name: str,
+    branch_name: str | None,
     thread_id: int,
     message_id: int,
     is_review_thread: bool = False,
@@ -227,6 +227,14 @@ def fan_out_chat_message(
     (state='watching', i.e. full stream) get a churn-class
     `chat_message_in_participated_thread`. The two are union'd so a user
     who is both gets only the personal-direct row.
+
+    v0.5.0: `branch_name` may be None — that is the PR-less per-RFC
+    discussion shape (`threads.branch_name IS NULL`, §5). The
+    notifications row carries the null through; the inbox prose renders
+    identically whether the chat lives on a branch or on the RFC's
+    discussion surface, and the §15.7 reconciler keys on
+    (rfc_slug, branch_name) so a null branch correctly matches the
+    PR-less discussion's eventual chat-seen-equivalent advance.
     """
     _bump_auto_watch(actor_user_id, rfc_slug)