backend: inbox table + endpoints, mention-triggered inbox population

- Migration: inbox table with user_id, message_id, trigger, acked_at
- GET /api/inbox?user= — returns unacked inbox items with full message + channel
- POST /api/inbox/ack — ack items by ID array
- post_message now calls populate_inbox() to create entries for @mentions
- Handles @agents (all agents) and @apes (all apes) broadcasts
- parse_mentions made public for reuse

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.claude/scheduled_tasks.lock

@@ -0,0 +1 @@
+{"sessionId":"6df01b4f-363c-489f-b71c-23531c8f6831","pid":73181,"acquiredAt":1774816757211}