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/libsql/conversations.rs

@@ -67,19 +67,20 @@ impl ConversationStore for LibSqlBackend {
         channel: &str,
         user_id: &str,
         thread_id: Option<&str>,
+        source_channel: Option<&str>,
     ) -> Result<bool, DatabaseError> {
         let conn = self.connect().await?;
         let now = fmt_ts(&Utc::now());
         let affected = conn
             .execute(
             r#"
-                INSERT INTO conversations (id, channel, user_id, thread_id, started_at, last_activity)
-                VALUES (?1, ?2, ?3, ?4, ?5, ?5)
+                INSERT INTO conversations (id, channel, user_id, thread_id, source_channel, started_at, last_activity)
+                VALUES (?1, ?2, ?3, ?4, ?5, ?6, ?6)
                 ON CONFLICT (id) DO UPDATE SET last_activity = excluded.last_activity
                 WHERE conversations.user_id = excluded.user_id
                   AND conversations.channel = excluded.channel
                 "#,
-            params![id.to_string(), channel, user_id, opt_text(thread_id), now],
+            params![id.to_string(), channel, user_id, opt_text(thread_id), opt_text(source_channel), now],
         )
             .await
             .map_err(|e| DatabaseError::Query(e.to_string()))?;
@@ -565,6 +566,28 @@ impl ConversationStore for LibSqlBackend {
             .map_err(|e| DatabaseError::Query(e.to_string()))?;
         Ok(found.is_some())
     }
+
+    async fn get_conversation_source_channel(
+        &self,
+        conversation_id: Uuid,
+    ) -> Result<Option<String>, DatabaseError> {
+        let conn = self.connect().await?;
+        let mut rows = conn
+            .query(
+                "SELECT source_channel FROM conversations WHERE id = ?1",
+                params![conversation_id.to_string()],
+            )
+            .await
+            .map_err(|e| DatabaseError::Query(e.to_string()))?;
+        match rows
+            .next()
+            .await
+            .map_err(|e| DatabaseError::Query(e.to_string()))?
+        {
+            Some(row) => Ok(get_opt_text(&row, 0)),
+            None => Ok(None),
+        }
+    }
 }
 
 #[cfg(test)]