Implement BaseModel class, TimestampMixin, TzDateTime type; cleanup Task model

tofu_api/common/database/mixins.py

@@ -0,0 +1,23 @@
+from datetime import datetime
+
+from sqlalchemy import Column, func
+from sqlalchemy.orm import declarative_mixin
+
+from tofu_api.common.database import Col
+from tofu_api.common.database.types import TzDateTime
+
+__all__ = [
+    'TimestampMixin'
+]
+
+
+@declarative_mixin
+class TimestampMixin:
+    """
+    Mixin for database models that provides the "created_at" and "modified_at" columns.
+    """
+    # Created timestamp (automatically set to NOW() once on object creation)
+    created_at: Col[datetime] = Column(TzDateTime, nullable=False, server_default=func.now())
+
+    # Modified timestamp (automatically set to NOW() on each update)
+    modified_at: Col[datetime] = Column(TzDateTime, nullable=False, server_default=func.now(), onupdate=func.now())