chore(backend): repair test suite and bring ruff to green

The suite hadn't run green since the routes refactor:
- test_profile_duplicate_names.py imported the pre-refactor module
  layout and broke collection; now imports backend.services.profiles
- tests/conftest.py puts the repo root and backend dir on sys.path so
  files collect standalone instead of depending on run order
- test_cors.py tested a hand-copied mirror of the origin list that had
  drifted from app.py (missing http://tauri.localhost); it now builds
  the app via the real create_app() factory
- test_progress.py simulated a 1KB download, below the tracker's 1MB
  reporting threshold; simulation raised to 5MB
- slow/timeout markers registered in pyproject

Ruff: ~900 violations auto-fixed (typing modernization, import
sorting, unused imports, whitespace). The remaining rules are baselined
in pyproject.toml with per-rule counts to burn down, plus per-file
carve-outs for deliberate env-before-import ordering. ruff check is
now clean; suite is 134 passed, 2 skipped.

backend/utils/hf_progress.py

@@ -2,11 +2,11 @@
 HuggingFace Hub download progress tracking.
 """
 
-from typing import Optional, Callable
-from contextlib import contextmanager
 import logging
-import threading
 import sys
+import threading
+from collections.abc import Callable
+from contextlib import contextmanager
 
 logger = logging.getLogger(__name__)
 
@@ -14,7 +14,7 @@ logger = logging.getLogger(__name__)
 class HFProgressTracker:
     """Tracks HuggingFace Hub download progress by intercepting tqdm."""
 
-    def __init__(self, progress_callback: Optional[Callable] = None, filter_non_downloads: bool = False):
+    def __init__(self, progress_callback: Callable | None = None, filter_non_downloads: bool = False):
         self.progress_callback = progress_callback
         self.filter_non_downloads = filter_non_downloads  # Only filter if True
         self._original_tqdm_class = None