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/main.py

@@ -5,10 +5,11 @@ entry point for development.
 """
 
 import argparse
+
 import uvicorn
 
-from .app import app  # noqa: F401 -- re-export for uvicorn "backend.main:app"
 from . import config, database
+from .app import app  # noqa: F401 -- re-export for uvicorn "backend.main:app"
 
 if __name__ == "__main__":
     parser = argparse.ArgumentParser(description="voicebox backend server")