refactor: remove dead code, deduplicate backends

Phase 1 - delete dead code:
- studio.py, migrate_add_instruct.py, utils/validation.py
- duplicate _profile_to_response in main.py, duplicate asyncio import
- pointless _get_profiles_dir/_get_generations_dir wrappers
- duplicate LANGUAGE_CODE_TO_NAME and WHISPER_HF_REPOS constants

Phase 2 - extract backends/base.py with shared utilities:
- is_model_cached() replaces 7 copy-pasted HF cache checks
- get_torch_device() replaces 5 device detection methods
- combine_voice_prompts() replaces 5 identical implementations
- model_load_progress() ctx manager replaces progress boilerplate in all backends
- patch_chatterbox_f32() replaces identical monkey-patches in both chatterbox backends

net -1078 lines across the backend

backend/history.py

@@ -15,11 +15,6 @@ from .database import Generation as DBGeneration, GenerationVersion as DBGenerat
 from . import config
 
 
-def _get_generations_dir() -> Path:
-    """Get generations directory from config."""
-    return config.get_generations_dir()
-
-
 def _get_versions_for_generation(generation_id: str, db: Session) -> tuple:
     """Get versions list and active version ID for a generation."""
     import json