fix(backend): critical bug fixes for agent termination and sprint validation

Bug Fixes:
- bulk_terminate_by_project now unassigns issues before terminating agents
  to prevent orphaned issue assignments
- PATCH /issues/{id} now validates sprint status - cannot assign issues
  to COMPLETED or CANCELLED sprints
- archive_project now performs cascading cleanup:
  - Terminates all active agent instances
  - Cancels all planned/active sprints
  - Unassigns issues from terminated agents

Added edge case tests for all fixed bugs (19 new tests total):
- TestBulkTerminateEdgeCases
- TestSprintStatusValidation
- TestArchiveProjectCleanup
- TestDataIntegrityEdgeCases (IDOR protection)

Coverage: 93% (1836 tests passing)

🤖 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

@@ -318,8 +318,29 @@ class CRUDAgentInstance(CRUDBase[AgentInstance, AgentInstanceCreate, AgentInstan
         *,
         project_id: UUID,
     ) -> int:
-        """Terminate all active instances in a project."""
+        """Terminate all active instances in a project.
+
+        Also unassigns all issues from these agents to prevent orphaned assignments.
+        """
         try:
+            # First, unassign all issues from agents in this project
+            # Get all agent IDs that will be terminated
+            agents_to_terminate = await db.execute(
+                select(AgentInstance.id).where(
+                    AgentInstance.project_id == project_id,
+                    AgentInstance.status != AgentStatus.TERMINATED,
+                )
+            )
+            agent_ids = [row[0] for row in agents_to_terminate.fetchall()]
+
+            # Unassign issues from these agents
+            if agent_ids:
+                await db.execute(
+                    update(Issue)
+                    .where(Issue.assigned_agent_id.in_(agent_ids))
+                    .values(assigned_agent_id=None)
+                )
+
             now = datetime.now(UTC)
             stmt = (
                 update(AgentInstance)