Refactor database models to use server defaults for boolean and timestamp fields

- Updated `TimestampMixin` to use `server_default` for `created_at` and `updated_at` fields.
- Modified various models in the `fa_classes`, `doc_types_dig`, `ports`, `units_of_measure`, `invoices`, `parts`, `trailers`, `licenses`, `tenants`, and `user_tenant` modules to set `server_default` for boolean fields and other relevant fields.
- Adjusted the `trailer_types` model to change the schema from `a76` to `public`.
- Implemented database migration execution during application startup in `main.py`.
- Removed old Alembic migration execution logic from the entrypoint script.
- Updated seed data for units of measure and removed unused seed files.

backend/api/v1/common/base_models.py

@@ -9,10 +9,10 @@ class TimestampMixin:
     """Mixin for common timestamp fields"""
 
     created_at: Mapped[datetime] = mapped_column(
-        DateTime, nullable=False, default=func.now()
+        DateTime, nullable=False, server_default=func.now()
     )
     updated_at: Mapped[datetime] = mapped_column(
-        DateTime, nullable=False, default=func.now(), onupdate=func.now()
+        DateTime, nullable=False, server_default=func.now(), onupdate=func.now()
     )
     deleted_at: Mapped[datetime | None] = mapped_column(DateTime, nullable=True)