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"

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/models/syndarix/agent_instance.py

@@ -62,7 +62,11 @@ class AgentInstance(Base, UUIDMixin, TimestampMixin):
 
     # Status tracking
     status: Column[AgentStatus] = Column(
-        Enum(AgentStatus, name="agent_status"),
+        Enum(
+            AgentStatus,
+            name="agent_status",
+            values_callable=lambda x: [e.value for e in x],
+        ),
         default=AgentStatus.IDLE,
         nullable=False,
         index=True,