M2 labels backend: labels + note_labels, CRUD, note-label set, filter

- Label + NoteLabel models; migration 0004 (labels unique per owner + note_labels
  join, cascade).
- /api/labels: list/create(idempotent)/rename(clash-checked)/delete, owner-scoped.
- PUT /api/notes/<id>/labels to set a note's labels (validated against owned).
- Note responses now include labels[] (merged via one explicit join query — no
  lazy relationship); GET /api/notes?...&label=<id> filters by label.
- DB-free auth-guard tests for labels endpoints.

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

src/thoughtsync/models/all.py

@@ -3,4 +3,4 @@
 Imported for side effects only (model registration on Base.metadata).
 """
 
-from . import group, note, settings, share, user  # noqa: F401
+from . import group, label, note, settings, share, user  # noqa: F401