refactor: split table models into Base/Table/Public for proper FastAPI serialization

Add ProductBase, ProductPublic, ProductWithInventory and
SkinConditionSnapshotBase, SkinConditionSnapshotPublic. Table models now inherit
from their Base counterpart and override JSON fields with sa_column. All
field_serializer hacks removed; FastAPI response models use the non-table Public
classes so Pydantic coerces raw DB dicts → typed models cleanly. ProductCreate
and SnapshotCreate now simply inherit their respective Base classes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/tests/test_skincare.py

@@ -112,9 +112,7 @@ def test_update_snapshot_concerns(client):
 
 
 def test_update_snapshot_not_found(client):
-    r = client.patch(
-        f"/skincare/{uuid.uuid4()}", json={"overall_state": "good"}
-    )
+    r = client.patch(f"/skincare/{uuid.uuid4()}", json={"overall_state": "good"})
     assert r.status_code == 404