chore(lint): fix all ruff errors — 0 warnings remaining
- B904 (55): add `from err` / `from None` to raise-in-except across 13 files
- F821 (1): add missing HTTPException import in routes_language_qc.py
- F841 (7): remove unused variable assignments (current_user, job_title, tts_provider, etc.)
- W293 (13): strip trailing whitespace from blank lines
- C416 (4): rewrite unnecessary dict comprehensions as dict()
- C401 (1): rewrite unnecessary generator as set comprehension
- E701 (4): split multi-statement lines in cost_tracker.py
- E741 (1): rename ambiguous `l` to `lang` in cloud_run_dispatch.py
- B007 (4): prefix unused loop variables with _ in tts.py, video_renderer.py
- I001 (1): sort imports in tasks/__init__.py (move stdlib to top)
- E402 (3): move threading/time/signals imports to top of tasks/__init__.py
- UP042 (9): replace (str, Enum) with StrEnum in all model/schema enums
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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