feat: implement background task architecture for code quality scans

- Add status fields to ArchitectureScan model (status, started_at,
  completed_at, error_message, progress_message)
- Create database migration for new status fields
- Create background task function execute_code_quality_scan()
- Update API to return 202 with job IDs and support polling
- Add code quality scans to unified BackgroundTasksService
- Integrate scans into background tasks API and page
- Implement frontend polling with 3-second interval
- Add progress banner showing scan status
- Users can navigate away while scans run in background
- Document the implementation in architecture docs

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

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

app/services/background_tasks_service.py

@@ -9,6 +9,7 @@ from datetime import UTC, datetime
 from sqlalchemy import case, desc, func
 from sqlalchemy.orm import Session
 
+from models.database.architecture_scan import ArchitectureScan
 from models.database.marketplace_import_job import MarketplaceImportJob
 from models.database.test_run import TestRun
 
@@ -124,6 +125,69 @@ class BackgroundTasksService:
             "avg_duration": round(stats.avg_duration or 0, 1),
         }
 
+    def get_code_quality_scans(
+        self, db: Session, status: str | None = None, limit: int = 50
+    ) -> list[ArchitectureScan]:
+        """Get code quality scans with optional status filter"""
+        query = db.query(ArchitectureScan)
+        if status:
+            query = query.filter(ArchitectureScan.status == status)
+        return query.order_by(desc(ArchitectureScan.timestamp)).limit(limit).all()
+
+    def get_running_scans(self, db: Session) -> list[ArchitectureScan]:
+        """Get currently running code quality scans"""
+        return (
+            db.query(ArchitectureScan)
+            .filter(ArchitectureScan.status.in_(["pending", "running"]))
+            .all()
+        )
+
+    def get_scan_stats(self, db: Session) -> dict:
+        """Get code quality scan statistics"""
+        today_start = datetime.now(UTC).replace(
+            hour=0, minute=0, second=0, microsecond=0
+        )
+
+        stats = db.query(
+            func.count(ArchitectureScan.id).label("total"),
+            func.sum(
+                case(
+                    (ArchitectureScan.status.in_(["pending", "running"]), 1), else_=0
+                )
+            ).label("running"),
+            func.sum(
+                case(
+                    (
+                        ArchitectureScan.status.in_(
+                            ["completed", "completed_with_warnings"]
+                        ),
+                        1,
+                    ),
+                    else_=0,
+                )
+            ).label("completed"),
+            func.sum(
+                case((ArchitectureScan.status == "failed", 1), else_=0)
+            ).label("failed"),
+            func.avg(ArchitectureScan.duration_seconds).label("avg_duration"),
+        ).first()
+
+        today_count = (
+            db.query(func.count(ArchitectureScan.id))
+            .filter(ArchitectureScan.timestamp >= today_start)
+            .scalar()
+            or 0
+        )
+
+        return {
+            "total": stats.total or 0,
+            "running": stats.running or 0,
+            "completed": stats.completed or 0,
+            "failed": stats.failed or 0,
+            "today": today_count,
+            "avg_duration": round(stats.avg_duration or 0, 1),
+        }
+
 
 # Singleton instance
 background_tasks_service = BackgroundTasksService()