fix(models): use enum values instead of names for PostgreSQL

Add values_callable to all enum columns so SQLAlchemy serializes using
the enum's .value (lowercase) instead of .name (uppercase). PostgreSQL
enum types defined in migrations use lowercase values.

Fixes: invalid input value for enum autonomy_level: "MILESTONE"

backend/app/models/syndarix/sprint.py

@@ -57,7 +57,11 @@ class Sprint(Base, UUIDMixin, TimestampMixin):
 
     # Status
     status: Column[SprintStatus] = Column(
-        Enum(SprintStatus, name="sprint_status"),
+        Enum(
+            SprintStatus,
+            name="sprint_status",
+            values_callable=lambda x: [e.value for e in x],
+        ),
         default=SprintStatus.PLANNED,
         nullable=False,
         index=True,