feat(api): add per-user health-aware upload rate limiting

Introduces a Redis-backed sliding-window rate limiter for upload
endpoints (/api/ui-upload, /api/process-url) that:
- Enforces per-user limits (default: 20 uploads / 60 s)
- Dynamically reduces limits under system stress (queue depth, CPU load)
- Returns 429 with Retry-After header when exceeded
- Fails open when Redis is unavailable
- Works with the existing client-side adaptive back-off

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

app/api/files.py

@@ -20,6 +20,7 @@ from sqlalchemy.orm import Session
 from app.auth import require_login
 from app.config import settings
 from app.database import get_db
+from app.middleware.upload_rate_limit import require_upload_rate_limit
 from app.models import FileProcessingStep, FileRecord, ProcessingLog
 from app.tasks.convert_to_pdf import convert_to_pdf
 from app.tasks.process_document import process_document
@@ -1280,7 +1281,12 @@ def _check_for_exact_duplicate(db: DbSession, target_path: str, safe_filename: s
 
 @router.post("/ui-upload")
 @require_login
-async def ui_upload(request: Request, db: DbSession, file: UploadFile = File(...)):
+async def ui_upload(
+    request: Request,
+    db: DbSession,
+    file: UploadFile = File(...),
+    _rate_ok: None = Depends(require_upload_rate_limit),
+):
     """Endpoint to accept a user-uploaded file and enqueue it for processing."""
     workdir = settings.workdir