refactor(B1): migrate to domain-driven project structure

Move all models/schemas/services/routers into app/domains/.
Keep backward-compat shims in old locations for imports.
Preserves domains/rendering/tasks.py from Phase A.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/domains/admin/router.py

@@ -0,0 +1,6 @@
+# Re-export from original routers.
+from app.api.routers.admin import router as admin_router
+from app.api.routers.analytics import router as analytics_router
+from app.api.routers.worker import router as worker_router
+
+__all__ = ["admin_router", "analytics_router", "worker_router"]