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/tasks.py

@@ -2,9 +2,8 @@
 Task tracking for active downloads and generations.
 """
 
-from typing import Optional, Dict, List
-from datetime import datetime
 from dataclasses import dataclass, field
+from datetime import datetime
 
 
 @dataclass
@@ -13,7 +12,7 @@ class DownloadTask:
     model_name: str
     status: str = "downloading"  # downloading, extracting, complete, error
     started_at: datetime = field(default_factory=datetime.utcnow)
-    error: Optional[str] = None
+    error: str | None = None
 
 
 @dataclass
@@ -27,29 +26,29 @@ class GenerationTask:
 
 class TaskManager:
     """Manages active downloads and generations."""
-    
+
     def __init__(self):
-        self._active_downloads: Dict[str, DownloadTask] = {}
-        self._active_generations: Dict[str, GenerationTask] = {}
-    
+        self._active_downloads: dict[str, DownloadTask] = {}
+        self._active_generations: dict[str, GenerationTask] = {}
+
     def start_download(self, model_name: str) -> None:
         """Mark a download as started."""
         self._active_downloads[model_name] = DownloadTask(
             model_name=model_name,
             status="downloading",
         )
-    
+
     def complete_download(self, model_name: str) -> None:
         """Mark a download as complete."""
         if model_name in self._active_downloads:
             del self._active_downloads[model_name]
-    
+
     def error_download(self, model_name: str, error: str) -> None:
         """Mark a download as failed."""
         if model_name in self._active_downloads:
             self._active_downloads[model_name].status = "error"
             self._active_downloads[model_name].error = error
-    
+
     def start_generation(self, task_id: str, profile_id: str, text: str) -> None:
         """Mark a generation as started."""
         text_preview = text[:50] + "..." if len(text) > 50 else text
@@ -58,20 +57,20 @@ class TaskManager:
             profile_id=profile_id,
             text_preview=text_preview,
         )
-    
+
     def complete_generation(self, task_id: str) -> None:
         """Mark a generation as complete."""
         if task_id in self._active_generations:
             del self._active_generations[task_id]
-    
-    def get_active_downloads(self) -> List[DownloadTask]:
+
+    def get_active_downloads(self) -> list[DownloadTask]:
         """Get all active downloads."""
         return list(self._active_downloads.values())
-    
-    def get_active_generations(self) -> List[GenerationTask]:
+
+    def get_active_generations(self) -> list[GenerationTask]:
         """Get all active generations."""
         return list(self._active_generations.values())
-    
+
     def cancel_download(self, model_name: str) -> bool:
         """Cancel/dismiss a download task (removes it from active list)."""
         return self._active_downloads.pop(model_name, None) is not None
@@ -84,14 +83,14 @@ class TaskManager:
     def is_download_active(self, model_name: str) -> bool:
         """Check if a download is active."""
         return model_name in self._active_downloads
-    
+
     def is_generation_active(self, task_id: str) -> bool:
         """Check if a generation is active."""
         return task_id in self._active_generations
 
 
 # Global task manager instance
-_task_manager: Optional[TaskManager] = None
+_task_manager: TaskManager | None = None
 
 
 def get_task_manager() -> TaskManager: