feat: add ApiKey model and migration 0027

Adds api_keys table with user_id FK, key_hash, key_prefix, scope
(read/write), last_used_at, revoked_at. SHA-256 hashed, soft-delete.

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

src/fabledassistant/models/__init__.py

@@ -40,3 +40,4 @@ from fabledassistant.models.share import NoteShare, ProjectShare  # noqa: E402,
 from fabledassistant.models.notification import Notification  # noqa: E402, F401
 from fabledassistant.models.rss_feed import RssFeed, RssItem  # noqa: E402, F401
 from fabledassistant.models.weather_cache import WeatherCache  # noqa: E402, F401
+from fabledassistant.models.api_key import ApiKey  # noqa: E402, F401