fix(backend): race condition fixes for task completion and sprint operations

## Changes

### agent_instance.py - Task Completion Counter Race Condition
- Changed `record_task_completion()` from read-modify-write pattern to
  atomic SQL UPDATE
- Previously: Read instance → increment in Python memory → write back
- Now: Uses `UPDATE ... SET tasks_completed = tasks_completed + 1`
- Prevents lost updates when multiple concurrent task completions occur

### sprint.py - Row-Level Locking for Sprint Operations
- Added `with_for_update()` to `complete_sprint()` to prevent race
  conditions during velocity calculation
- Added `with_for_update()` to `cancel_sprint()` for consistency
- Ensures atomic check-and-update for sprint status changes

## Impact
These fixes prevent:
- Counter metrics being lost under concurrent load
- Data corruption during sprint completion
- Race conditions with concurrent sprint status changes

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

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

backend/app/crud/syndarix/agent_instance.py

@@ -249,23 +249,35 @@ class CRUDAgentInstance(CRUDBase[AgentInstance, AgentInstanceCreate, AgentInstan
         tokens_used: int,
         cost_incurred: Decimal,
     ) -> AgentInstance | None:
-        """Record a completed task and update metrics."""
+        """Record a completed task and update metrics.
+
+        Uses atomic SQL UPDATE to prevent lost updates under concurrent load.
+        This avoids the read-modify-write race condition that occurs when
+        multiple task completions happen simultaneously.
+        """
         try:
+            now = datetime.now(UTC)
+
+            # Use atomic SQL UPDATE to increment counters without race conditions
+            # This is safe for concurrent updates - no read-modify-write pattern
             result = await db.execute(
-                select(AgentInstance).where(AgentInstance.id == instance_id)
+                update(AgentInstance)
+                .where(AgentInstance.id == instance_id)
+                .values(
+                    tasks_completed=AgentInstance.tasks_completed + 1,
+                    tokens_used=AgentInstance.tokens_used + tokens_used,
+                    cost_incurred=AgentInstance.cost_incurred + cost_incurred,
+                    last_activity_at=now,
+                    updated_at=now,
+                )
+                .returning(AgentInstance)
             )
             instance = result.scalar_one_or_none()
 
             if not instance:
                 return None
 
-            instance.tasks_completed += 1
-            instance.tokens_used += tokens_used
-            instance.cost_incurred += cost_incurred
-            instance.last_activity_at = datetime.now(UTC)
-
             await db.commit()
-            await db.refresh(instance)
             return instance
         except Exception as e:
             await db.rollback()