fix(timestamps): callable default so each row gets a fresh timestamp

TimestampMixin was setting default=datetime.now(UTC) — calling the
function once at module import time. Every INSERT (and every UPDATE
via onupdate) since the app last restarted got stamped with the same
process-start timestamp, silently breaking created_at / updated_at on
every table that uses the mixin (customers, stores, cards, users,
orders, etc.).

Pass the _utc_now callable instead so SQLAlchemy invokes it per row.
Forward fix only — historical rows on prod since the most recent app
restart all read as the start-time microsecond and will need a
separate decision on whether to backfill.

Found while debugging why a customer + their loyalty card had
microsecond-identical created_at timestamps despite being created
through separate service calls.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

models/database/base.py

@@ -5,14 +5,20 @@ from sqlalchemy import Column, DateTime, ForeignKey, Integer
 from app.core.database import Base
 
 
+def _utc_now() -> datetime:
+    return datetime.now(UTC)
+
+
 class TimestampMixin:
     """Mixin to add created_at and updated_at timestamps to models"""
 
-    created_at = Column(DateTime, default=datetime.now(UTC), nullable=False)
+    # Pass the callable, not its result. Otherwise the default is evaluated
+    # once at module import and every row gets the same timestamp.
+    created_at = Column(DateTime, default=_utc_now, nullable=False)
     updated_at = Column(
         DateTime,
-        default=datetime.now(UTC),
-        onupdate=datetime.now(UTC),
+        default=_utc_now,
+        onupdate=_utc_now,
         nullable=False,
     )