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/routes/health.py

@@ -6,13 +6,11 @@ import signal
 from pathlib import Path
 
 import torch
-from fastapi import APIRouter, Depends
+from fastapi import APIRouter
 from fastapi.responses import FileResponse
-from sqlalchemy.orm import Session
 
 from .. import config, models
 from ..services import tts
-from ..database import get_db
 from ..utils.platform_detect import get_backend_type, is_amd_gpu_windows
 
 router = APIRouter()
@@ -56,9 +54,10 @@ async def watchdog_disable():
 @router.get("/health", response_model=models.HealthResponse)
 async def health():
     """Health check endpoint."""
-    from huggingface_hub import constants as hf_constants
     from pathlib import Path
 
+    from huggingface_hub import constants as hf_constants
+
     tts_model = tts.get_tts_model()
     backend_type = get_backend_type()