fix: Handle encrypted image upload for Matrix rooms (MAT-9)

Upload with encrypt=True and filesize param. Handle UploadError
gracefully. Use m.file encrypted format when encryption keys returned.

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

bot.py

@@ -974,20 +974,39 @@ class Bot:
 
     async def _send_image(self, room_id: str, image_bytes: bytes, mime_type: str, filename: str):
         """Upload image to Matrix homeserver and send as m.image event."""
-        upload_resp, _ = await self.client.upload(
+        from nio import UploadResponse
+        upload_resp, maybe_keys = await self.client.upload(
             data_provider=io.BytesIO(image_bytes),
             content_type=mime_type,
             filename=filename,
+            filesize=len(image_bytes),
+            encrypt=True,
         )
+        if not isinstance(upload_resp, UploadResponse):
+            logger.error("Image upload failed: %s", upload_resp)
+            await self._send_text(room_id, "Sorry, I couldn't upload the generated image.")
+            return
+
+        content = {
+            "msgtype": "m.image",
+            "body": filename,
+            "info": {"mimetype": mime_type, "size": len(image_bytes)},
+        }
+        if maybe_keys:
+            content["file"] = {
+                "url": upload_resp.content_uri,
+                "key": maybe_keys["key"],
+                "iv": maybe_keys["iv"],
+                "hashes": maybe_keys["hashes"],
+                "v": maybe_keys["v"],
+            }
+        else:
+            content["url"] = upload_resp.content_uri
+
         await self.client.room_send(
             room_id,
             message_type="m.room.message",
-            content={
-                "msgtype": "m.image",
-                "body": filename,
-                "url": upload_resp.content_uri,
-                "info": {"mimetype": mime_type, "size": len(image_bytes)},
-            },
+            content=content,
         )
 
     async def _send_text(self, room_id: str, text: str):