fix(websocket): update message metadata in-place without full chat reload

Instead of reloading the entire message list when echo data arrives,
now updates only the affected message elements in the DOM:

- Add data-msg-id attribute to message wrappers for targeted lookup
- Add GET /api/messages/<id>/meta endpoint returning metadata for a
  single message (computes pkt_payload, looks up echoes, analyzer URL)
- Replace loadMessages() echo handler with refreshMessagesMeta() that
  finds messages missing metadata and updates them individually
- Fix path_len=0 treated as falsy (use ?? instead of ||)

Flow: message appears instantly via WebSocket (with SNR + hops), then
~2s later echo data triggers targeted meta fetch → route info and
analyzer button appear smoothly without any chat window reload.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/database.py

@@ -328,6 +328,13 @@ class Database:
             )
             return cursor.lastrowid
 
+    def get_channel_message_by_id(self, msg_id: int) -> Optional[Dict]:
+        with self._connect() as conn:
+            row = conn.execute(
+                "SELECT * FROM channel_messages WHERE id = ?", (msg_id,)
+            ).fetchone()
+            return dict(row) if row else None
+
     def get_channel_messages(self, channel_idx: int = None, limit: int = 50,
                               offset: int = 0, days: int = None) -> List[Dict]:
         with self._connect() as conn: