fix(signal): use id instead of attachmentId in getAttachment RPC

Cherry-picked from PR #2365 by @xerpert.

Three bugs preventing Signal image attachments from being processed:
1. signal-cli getAttachment RPC expects 'id', not 'attachmentId'
2. signal-cli daemon returns dict {"data": "base64..."} not raw base64
3. MessageType.IMAGE doesn't exist — correct enum is MessageType.PHOTO

gateway/platforms/signal.py

@@ -478,7 +478,7 @@ class SignalAdapter(BasePlatformAdapter):
             if any(mt.startswith("audio/") for mt in media_types):
                 msg_type = MessageType.VOICE
             elif any(mt.startswith("image/") for mt in media_types):
-                msg_type = MessageType.IMAGE
+                msg_type = MessageType.PHOTO
 
         # Parse timestamp from envelope data (milliseconds since epoch)
         ts_ms = envelope_data.get("timestamp", 0)
@@ -519,6 +519,13 @@ class SignalAdapter(BasePlatformAdapter):
         if not result:
             return None, ""
 
+        # Handle dict response (signal-cli returns {"data": "base64..."})
+        if isinstance(result, dict):
+            result = result.get("data")
+            if not result:
+                logger.warning("Signal: attachment response missing 'data' key")
+                return None, ""
+
         # Result is base64-encoded file content
         raw_data = base64.b64decode(result)
         ext = _guess_extension(raw_data)