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/project.py

@@ -5,13 +5,15 @@ import logging
 from typing import Any
 from uuid import UUID
 
-from sqlalchemy import func, or_, select
+from datetime import UTC, datetime
+
+from sqlalchemy import func, or_, select, update
 from sqlalchemy.exc import IntegrityError
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.crud.base import CRUDBase
 from app.models.syndarix import AgentInstance, Issue, Project, Sprint
-from app.models.syndarix.enums import ProjectStatus, SprintStatus
+from app.models.syndarix.enums import AgentStatus, ProjectStatus, SprintStatus
 from app.schemas.syndarix import ProjectCreate, ProjectUpdate
 
 logger = logging.getLogger(__name__)
@@ -283,7 +285,13 @@ class CRUDProject(CRUDBase[Project, ProjectCreate, ProjectUpdate]):
         *,
         project_id: UUID,
     ) -> Project | None:
-        """Archive a project by setting status to ARCHIVED."""
+        """Archive a project by setting status to ARCHIVED.
+
+        This also performs cascading cleanup:
+        - Terminates all active agent instances
+        - Cancels all planned/active sprints
+        - Unassigns issues from terminated agents
+        """
         try:
             result = await db.execute(
                 select(Project).where(Project.id == project_id)
@@ -293,9 +301,63 @@ class CRUDProject(CRUDBase[Project, ProjectCreate, ProjectUpdate]):
             if not project:
                 return None
 
+            now = datetime.now(UTC)
+
+            # 1. 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()]
+
+            # 2. Unassign issues from these agents to prevent orphaned assignments
+            if agent_ids:
+                await db.execute(
+                    update(Issue)
+                    .where(Issue.assigned_agent_id.in_(agent_ids))
+                    .values(assigned_agent_id=None)
+                )
+
+            # 3. Terminate all active agents
+            await db.execute(
+                update(AgentInstance)
+                .where(
+                    AgentInstance.project_id == project_id,
+                    AgentInstance.status != AgentStatus.TERMINATED,
+                )
+                .values(
+                    status=AgentStatus.TERMINATED,
+                    terminated_at=now,
+                    current_task=None,
+                    session_id=None,
+                    updated_at=now,
+                )
+            )
+
+            # 4. Cancel all planned/active sprints
+            await db.execute(
+                update(Sprint)
+                .where(
+                    Sprint.project_id == project_id,
+                    Sprint.status.in_([SprintStatus.PLANNED, SprintStatus.ACTIVE]),
+                )
+                .values(
+                    status=SprintStatus.CANCELLED,
+                    updated_at=now,
+                )
+            )
+
+            # 5. Archive the project
             project.status = ProjectStatus.ARCHIVED
             await db.commit()
             await db.refresh(project)
+
+            logger.info(
+                f"Archived project {project_id}: terminated agents={len(agent_ids)}"
+            )
+
             return project
         except Exception as e:
             await db.rollback()