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/chat.py

@@ -168,10 +168,15 @@ def _fan_out_chat(thread_id: int, author_user_id: int, message_id: int) -> None:
         ).fetchone()
         if pr_row:
             pr_number = pr_row["pr_number"]
+    # v0.5.0 (§5 / §10 — PR-less discussion): a thread with
+    # branch_name IS NULL is scoped to the RFC's main view. Pass None
+    # through to the notify chokepoint so the notifications row keeps
+    # `branch_name` null — coercing it to "main" would misroute the
+    # §15.7 chat-seen reconciler (which keys on branch_name).
     notify.fan_out_chat_message(
         actor_user_id=author_user_id,
         rfc_slug=row["rfc_slug"],
-        branch_name=row["branch_name"] or "main",
+        branch_name=row["branch_name"],
         thread_id=thread_id,
         message_id=message_id,
         is_review_thread=(row["thread_kind"] == "review"),