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/columns.py

@@ -0,0 +1,58 @@
+"""SQLAlchemy column mixins for common column patterns."""
+
+import uuid
+from datetime import datetime
+
+from sqlalchemy import DateTime, Uuid, text
+from sqlalchemy.orm import Mapped, mapped_column
+
+__all__ = [
+    "UUIDMixin",
+    "UUIDv7Mixin",
+    "CreatedAtMixin",
+    "UpdatedAtMixin",
+    "TimestampMixin",
+]
+
+
+class UUIDMixin:
+    """Mixin that adds a UUID primary key auto-generated by the database."""
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        Uuid,
+        primary_key=True,
+        server_default=text("gen_random_uuid()"),
+    )
+
+
+class UUIDv7Mixin:
+    """Mixin that adds a UUIDv7 primary key auto-generated by the database."""
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        Uuid,
+        primary_key=True,
+        server_default=text("uuidv7()"),
+    )
+
+
+class CreatedAtMixin:
+    """Mixin that adds a ``created_at`` timestamp column."""
+
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        server_default=text("clock_timestamp()"),
+    )
+
+
+class UpdatedAtMixin:
+    """Mixin that adds an ``updated_at`` timestamp column."""
+
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        server_default=text("clock_timestamp()"),
+        onupdate=text("clock_timestamp()"),
+    )
+
+
+class TimestampMixin(CreatedAtMixin, UpdatedAtMixin):
+    """Mixin that combines ``created_at`` and ``updated_at`` timestamp columns."""