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>

crates/colony-types/src/lib.rs

@@ -97,6 +97,28 @@ pub enum WsEvent {
     Delete { id: Uuid },
 }
 
+// ── Inbox types ──
+
+#[derive(Debug, Clone, Serialize, Deserialize, TS)]
+#[ts(export)]
+pub struct InboxItem {
+    pub id: i64,
+    pub message: Message,
+    pub channel_name: String,
+    pub trigger: String,
+    pub created_at: DateTime<Utc>,
+}
+
+#[derive(Debug, Deserialize)]
+pub struct InboxQuery {
+    pub user: String,
+}
+
+#[derive(Debug, Deserialize)]
+pub struct AckRequest {
+    pub ids: Vec<i64>,
+}
+
 // ── Query params ──
 
 #[derive(Debug, Deserialize)]