feat(queue): add queue monitoring dashboard and pending banner on files page

- Add /api/queue/stats endpoint with Redis queue lengths, Celery worker
  inspection, and DB processing summaries
- Add /api/queue/pending-count lightweight endpoint for the files page banner
- Add /admin/queue admin-only view with auto-refreshing queue dashboard
- Add queue pending banner on /files page showing queued/processing count
- Add Queue Monitor link to admin dropdown in navigation (desktop + mobile)
- Add comprehensive tests for all new endpoints and views

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/views/queue.py

@@ -0,0 +1,32 @@
+"""
+Queue monitoring view for the admin dashboard.
+"""
+
+import logging
+
+from fastapi import Depends, Request
+from sqlalchemy.orm import Session
+
+from app.views.base import APIRouter, get_db, require_login, templates
+from app.views.settings import require_admin_access
+
+logger = logging.getLogger(__name__)
+router = APIRouter()
+
+
+@router.get("/admin/queue")
+@require_login
+@require_admin_access
+async def queue_dashboard(request: Request, db: Session = Depends(get_db)):
+    """
+    Queue monitoring dashboard — admin only.
+
+    Displays Celery/Redis queue statistics and database processing summaries
+    so administrators can monitor the document processing pipeline.
+    """
+    return templates.TemplateResponse(
+        "queue_dashboard.html",
+        {
+            "request": request,
+        },
+    )