feat: add WatchedFieldsMixin (#148)

* feat/add WatchedFieldsMixin and watch_fields decorator for field-change monitoring

* docs: add WatchedFieldsMixin

* feat: add on_event, on_create and on_delete

* docs: update README

src/fastapi_toolsets/models/__init__.py

@@ -0,0 +1,21 @@
+"""SQLAlchemy model mixins for common column patterns."""
+
+from .columns import (
+    CreatedAtMixin,
+    TimestampMixin,
+    UUIDMixin,
+    UUIDv7Mixin,
+    UpdatedAtMixin,
+)
+from .watched import ModelEvent, WatchedFieldsMixin, watch
+
+__all__ = [
+    "ModelEvent",
+    "UUIDMixin",
+    "UUIDv7Mixin",
+    "CreatedAtMixin",
+    "UpdatedAtMixin",
+    "TimestampMixin",
+    "WatchedFieldsMixin",
+    "watch",
+]