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/tests/conftest.py

@@ -0,0 +1,17 @@
+"""Shared test setup.
+
+The suite mixes flat imports (``from utils.progress import ...``) with
+package imports (``from backend import config``). Both the repo root and
+the backend directory go on ``sys.path`` here so every test file collects
+on its own, regardless of which file loads first.
+"""
+
+import sys
+from pathlib import Path
+
+BACKEND_DIR = Path(__file__).resolve().parent.parent
+REPO_ROOT = BACKEND_DIR.parent
+
+for _path in (str(BACKEND_DIR), str(REPO_ROOT)):
+    if _path not in sys.path:
+        sys.path.insert(0, _path)