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>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "rfc-app-frontend",
   "private": true,
-  "version": "0.4.0",
+  "version": "0.5.0",
   "type": "module",
   "scripts": {
     "dev": "vite",