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/database/__init__.py

@@ -6,8 +6,8 @@ without changing any importers.
 """
 
 from .models import (
-    Base,
     AudioChannel,
+    Base,
     Capture,
     CaptureSettings,
     ChannelDeviceMapping,
@@ -24,12 +24,12 @@ from .models import (
     StoryItem,
     VoiceProfile,
 )
-from .session import engine, SessionLocal, _db_path, init_db, get_db
+from .session import SessionLocal, _db_path, engine, get_db, init_db
 
 __all__ = [
+    "AudioChannel",
     # Models
     "Base",
-    "AudioChannel",
     "Capture",
     "CaptureSettings",
     "ChannelDeviceMapping",
@@ -42,13 +42,13 @@ __all__ = [
     "ProfileChannelMapping",
     "ProfileSample",
     "Project",
+    "SessionLocal",
     "Story",
     "StoryItem",
     "VoiceProfile",
+    "_db_path",
     # Session
     "engine",
-    "SessionLocal",
-    "_db_path",
-    "init_db",
     "get_db",
+    "init_db",
 ]