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/mcp_bindings.py

@@ -6,7 +6,7 @@ column is the same value the MCP client sends in ``X-Voicebox-Client-Id``
 (or the stdio shim pulls from ``VOICEBOX_CLIENT_ID``).
 """
 
-from datetime import datetime, timezone
+from datetime import UTC, datetime
 
 from fastapi import APIRouter, Depends, HTTPException
 from sqlalchemy.orm import Session
@@ -15,7 +15,6 @@ from .. import models
 from ..database import get_db
 from ..database.models import MCPClientBinding
 
-
 router = APIRouter()
 
 
@@ -56,7 +55,7 @@ async def upsert_mcp_binding(
     row.profile_id = data.profile_id
     row.default_engine = data.default_engine
     row.default_personality = data.default_personality
-    row.updated_at = datetime.now(timezone.utc)
+    row.updated_at = datetime.now(UTC)
     db.commit()
     db.refresh(row)
     return models.MCPClientBindingResponse.model_validate(row)