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/api/routes/issues.py

@@ -31,7 +31,13 @@ from app.crud.syndarix.agent_instance import agent_instance as agent_instance_cr
 from app.crud.syndarix.issue import issue as issue_crud
 from app.crud.syndarix.project import project as project_crud
 from app.crud.syndarix.sprint import sprint as sprint_crud
-from app.models.syndarix.enums import AgentStatus, IssuePriority, IssueStatus, SyncStatus
+from app.models.syndarix.enums import (
+    AgentStatus,
+    IssuePriority,
+    IssueStatus,
+    SprintStatus,
+    SyncStatus,
+)
 from app.models.user import User
 from app.schemas.common import (
     MessageResponse,
@@ -550,7 +556,7 @@ async def update_issue(
                 field="assigned_agent_id",
             )
 
-    # Validate sprint if being updated (IDOR prevention)
+    # Validate sprint if being updated (IDOR prevention and status validation)
     if issue_in.sprint_id is not None:
         sprint = await sprint_crud.get(db, id=issue_in.sprint_id)
         if not sprint:
@@ -564,6 +570,13 @@ async def update_issue(
                 error_code=ErrorCode.VALIDATION_ERROR,
                 field="sprint_id",
             )
+        # Cannot add issues to completed or cancelled sprints
+        if sprint.status in [SprintStatus.COMPLETED, SprintStatus.CANCELLED]:
+            raise ValidationException(
+                message=f"Cannot add issues to sprint with status '{sprint.status.value}'",
+                error_code=ErrorCode.VALIDATION_ERROR,
+                field="sprint_id",
+            )
 
     try:
         updated_issue = await issue_crud.update(db, db_obj=issue, obj_in=issue_in)