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/profiles.py

@@ -58,11 +58,6 @@ def _profile_to_response(
     )
 
 
-def _get_profiles_dir() -> Path:
-    """Get profiles directory from config."""
-    return config.get_profiles_dir()
-
-
 async def create_profile(
     data: VoiceProfileCreate,
     db: Session,
@@ -100,7 +95,7 @@ async def create_profile(
     db.refresh(db_profile)
 
     # Create profile directory
-    profile_dir = _get_profiles_dir() / db_profile.id
+    profile_dir = config.get_profiles_dir() / db_profile.id
     profile_dir.mkdir(parents=True, exist_ok=True)
 
     return _profile_to_response(db_profile)
@@ -136,7 +131,7 @@ async def add_profile_sample(
     
     # Create sample ID and directory
     sample_id = str(uuid.uuid4())
-    profile_dir = _get_profiles_dir() / profile_id
+    profile_dir = config.get_profiles_dir() / profile_id
     profile_dir.mkdir(parents=True, exist_ok=True)
     
     # Copy audio file to profile directory
@@ -316,7 +311,7 @@ async def delete_profile(
     db.commit()
     
     # Delete profile directory
-    profile_dir = _get_profiles_dir() / profile_id
+    profile_dir = config.get_profiles_dir() / profile_id
     if profile_dir.exists():
         shutil.rmtree(profile_dir)
     
@@ -516,7 +511,7 @@ async def upload_avatar(
         ext = ext_map.get(img_format, '.png')
 
     # Save processed image to profile directory
-    profile_dir = _get_profiles_dir() / profile_id
+    profile_dir = config.get_profiles_dir() / profile_id
     profile_dir.mkdir(parents=True, exist_ok=True)
     output_path = profile_dir / f"avatar{ext}"