M2 attachments: image upload + owner-scoped media serving

- Migration 0007: note_attachments (path/mime/size). Upload POST
  /api/notes/<id>/attachments (multipart, png/jpeg/gif/webp, 12MB cap via
  MAX_CONTENT_LENGTH) stored under Config.media_root() (first use of DATA_DIR);
  owner/ACL-scoped GET serves the file (nosniff); DELETE removes row + file.
  Note responses include attachments[].
- Frontend: notes store uploadAttachment (FormData)/deleteAttachment; editor
  image button + paste-to-upload + thumbnail grid with remove; card shows the
  first image as a cover.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

src/thoughtsync/app.py

@@ -28,6 +28,7 @@ def create_app() -> Quart:
     app.config["SESSION_COOKIE_HTTPONLY"] = True
     app.config["SESSION_COOKIE_SAMESITE"] = "Lax"
     app.config["PERMANENT_SESSION_LIFETIME"] = timedelta(days=30)
+    app.config["MAX_CONTENT_LENGTH"] = 12 * 1024 * 1024  # image-upload body cap
 
     app.register_blueprint(auth_bp)
     app.register_blueprint(notes_bp)