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/issue.py

@@ -59,7 +59,9 @@ class Issue(Base, UUIDMixin, TimestampMixin):
 
     # Issue type (Epic, Story, Task, Bug)
     type: Column[IssueType] = Column(
-        Enum(IssueType, name="issue_type"),
+        Enum(
+            IssueType, name="issue_type", values_callable=lambda x: [e.value for e in x]
+        ),
         default=IssueType.TASK,
         nullable=False,
         index=True,
@@ -78,14 +80,22 @@ class Issue(Base, UUIDMixin, TimestampMixin):
 
     # Status and priority
     status: Column[IssueStatus] = Column(
-        Enum(IssueStatus, name="issue_status"),
+        Enum(
+            IssueStatus,
+            name="issue_status",
+            values_callable=lambda x: [e.value for e in x],
+        ),
         default=IssueStatus.OPEN,
         nullable=False,
         index=True,
     )
 
     priority: Column[IssuePriority] = Column(
-        Enum(IssuePriority, name="issue_priority"),
+        Enum(
+            IssuePriority,
+            name="issue_priority",
+            values_callable=lambda x: [e.value for e in x],
+        ),
         default=IssuePriority.MEDIUM,
         nullable=False,
         index=True,
@@ -132,7 +142,11 @@ class Issue(Base, UUIDMixin, TimestampMixin):
 
     # Sync status with external tracker
     sync_status: Column[SyncStatus] = Column(
-        Enum(SyncStatus, name="sync_status"),
+        Enum(
+            SyncStatus,
+            name="sync_status",
+            values_callable=lambda x: [e.value for e in x],
+        ),
         default=SyncStatus.SYNCED,
         nullable=False,
         # Note: Index defined in __table_args__ as ix_issues_sync_status