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/tests/test_profile_duplicate_names.py

@@ -45,8 +45,8 @@ def test_db():
 @pytest.fixture
 def mock_profiles_dir(monkeypatch, tmp_path):
     """Mock the profiles directory to use a temporary path."""
-    import profiles
-    monkeypatch.setattr(profiles, '_get_profiles_dir', lambda: tmp_path)
+    from backend import config
+    monkeypatch.setattr(config, 'get_profiles_dir', lambda: tmp_path)
     return tmp_path