Backend: drop users router, backups service attribution, named dashboards (Slice 3)

Users router removed:
- Delete routers/users.py + users_impl.py (Jellyfin-backed user directory,
  Jellyfin-email message compose, Jellyseerr enrichment).
- Drop orphaned get_jellyseerr_client dep from dependencies.py
  (get_user_id stays; used by dashboard/media/media_index_worker).
- clients/jellyseerr.py stays (still imported by widgets/sources.py).
- test_api.py TestUsers block + mock_jellyseerr fixture removed.

Backups service attribution:
- backup_jobs gains a nullable service_id column (PRAGMA migration).
- _resolve_backup_service_id helper: explicit service_id wins, else
  first-wins an enabled backups instance, else empty (backward-compat).
- Both report endpoints accept ?service_id= and persist it on the job.
- Dashboard summary + poller aggregate across all jobs unchanged.

Named dashboards backend:
- named_dashboards table (id, label, slug UNIQUE, sort_order, payload_json,
  timestamps) with full CRUD methods + _slugify/_unique_slug helpers.
- models/dashboards.py (NamedDashboardInput/NamedDashboard).
- routers/dashboards.py: GET/POST/PUT/DELETE /api/dashboards.
- Router registered in main.py.

Tests: test_dashboards.py (CRUD, slug collision, explicit slug, 404);
test_api.py trimmed. 271 backend tests pass (was 268; +6 dashboards -3
users); ruff clean.

Refs openspec/changes/services-as-hub-ia/ (spec R5/R6.1, tasks slice 3).

backend/src/media_library_viewer_api/routers/backups.py

@@ -15,7 +15,23 @@ from ..services.settings_store import SettingsStore, get_settings_store
 router = APIRouter(prefix="/api/backups", tags=["backups"])
 
 
-def _get_or_create_job(store: SettingsStore, report: BackupReportRequest) -> dict[str, Any]:
+def _resolve_backup_service_id(store: SettingsStore, explicit: str | None = None) -> str:
+    """Return the service_id for backup attribution.
+
+    First-wins: if no explicit service_id is given, pick the first enabled
+    ``backups`` service instance (spec R6.1). Returns an empty string when
+    none is configured (backward-compatible with pre-service reports).
+    """
+    if explicit:
+        return explicit
+    candidates = store.list_services("backups")
+    for svc in candidates:
+        if svc.get("enabled"):
+            return svc["id"]
+    return ""
+
+
+def _get_or_create_job(store: SettingsStore, report: BackupReportRequest, service_id: str = "") -> dict[str, Any]:
     job = store.get_backup_job_by_name(report.name)
     if not job:
         job = store.upsert_backup_job(
@@ -24,6 +40,7 @@ def _get_or_create_job(store: SettingsStore, report: BackupReportRequest) -> dic
                 "source": report.source,
                 "target": report.target,
                 "schedule_interval_seconds": report.schedule_interval_seconds,
+                "service_id": service_id,
             }
         )
     elif report.schedule_interval_seconds:
@@ -34,6 +51,7 @@ def _get_or_create_job(store: SettingsStore, report: BackupReportRequest) -> dic
                 "source": report.source,
                 "target": report.target,
                 "schedule_interval_seconds": report.schedule_interval_seconds,
+                "service_id": service_id,
             }
         )
         job = store.get_backup_job(job["id"])
@@ -43,10 +61,12 @@ def _get_or_create_job(store: SettingsStore, report: BackupReportRequest) -> dic
 @router.post("/report")
 def post_backup_report(
     report: BackupReportRequest,
+    service_id: str | None = None,
     store: SettingsStore = Depends(get_settings_store),
     _auth: str = Depends(require_api_key),
 ) -> BackupRunResponse:
-    job = _get_or_create_job(store, report)
+    resolved_service_id = _resolve_backup_service_id(store, service_id)
+    job = _get_or_create_job(store, report, resolved_service_id)
 
     # Check for duplicate (same job + started_at within 1s)
     existing_runs = store.list_backup_runs(job_id=job["id"], limit=5)
@@ -88,10 +108,12 @@ def post_backup_report(
 @router.post("/report/start")
 def post_backup_start(
     report: BackupReportRequest,
+    service_id: str | None = None,
     store: SettingsStore = Depends(get_settings_store),
     _auth: str = Depends(require_api_key),
 ) -> BackupRunResponse:
-    job = _get_or_create_job(store, report)
+    resolved_service_id = _resolve_backup_service_id(store, service_id)
+    job = _get_or_create_job(store, report, resolved_service_id)
 
     run_data = {
         "job_id": job["id"],