fix(security): persist source_channel to DB, harden cross-channel authorization

Address PR #1590 review feedback:

1. Persist source_channel to DB: Add source_channel column to conversations
   table in both PostgreSQL (V14 migration) and libSQL (incremental migration
   + base schema). Add get_conversation_source_channel trait method to
   ConversationStore with both backend implementations.

2. Fix hydrate_thread_from_db: Read source_channel from DB instead of
   stamping the requesting message's channel, preventing channel confusion
   after server restart.

3. Reject reserved WASM channel names: Validate that WASM channels cannot
   register as "web", "gateway", "cli", or "repl" to prevent authorization
   bypass via name spoofing.

4. Require pending_approval exists: Authorization check now verifies
   thread.pending_approval.is_some() before allowing approval-shaped messages
   to target a thread.

5. Fail-closed for None source_channel: Use "__bootstrap__" sentinel for
   bootstrap threads (authorized from any channel). None now means "deny by
   default" instead of "allow by default".

6. Extract and test authorization predicate: is_approval_authorized() helper
   with 6 unit tests covering same-channel, cross-channel blocked, web/gateway
   always allowed, None denied, and bootstrap sentinel.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

src/db/mod.rs

@@ -373,6 +373,7 @@ pub trait ConversationStore: Send + Sync {
         channel: &str,
         user_id: &str,
         thread_id: Option<&str>,
+        source_channel: Option<&str>,
     ) -> Result<bool, DatabaseError>;
     async fn list_conversations_with_preview(
         &self,
@@ -431,6 +432,11 @@ pub trait ConversationStore: Send + Sync {
         conversation_id: Uuid,
         user_id: &str,
     ) -> Result<bool, DatabaseError>;
+    /// Get the source_channel for a conversation (the channel that created it).
+    async fn get_conversation_source_channel(
+        &self,
+        conversation_id: Uuid,
+    ) -> Result<Option<String>, DatabaseError>;
 }
 
 #[async_trait]