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/utils/validation.py

@@ -1,66 +0,0 @@
-"""
-Input validation utilities.
-"""
-
-from typing import Tuple, Optional
-from pathlib import Path
-
-
-def validate_text(text: str, max_length: int = 5000) -> Tuple[bool, Optional[str]]:
-    """
-    Validate text input.
-    
-    Args:
-        text: Text to validate
-        max_length: Maximum length
-        
-    Returns:
-        Tuple of (is_valid, error_message)
-    """
-    if not text or not text.strip():
-        return False, "Text cannot be empty"
-    
-    if len(text) > max_length:
-        return False, f"Text too long (maximum {max_length} characters)"
-    
-    return True, None
-
-
-def validate_language(language: str) -> Tuple[bool, Optional[str]]:
-    """
-    Validate language code.
-
-    Supported languages for Qwen3-TTS:
-    Chinese, English, Japanese, Korean, German, French, Russian, Portuguese, Spanish, Italian
-
-    Args:
-        language: Language code
-
-    Returns:
-        Tuple of (is_valid, error_message)
-    """
-    valid_languages = ["zh", "en", "ja", "ko", "de", "fr", "ru", "pt", "es", "it"]
-    if language not in valid_languages:
-        return False, f"Invalid language (must be one of: {', '.join(valid_languages)})"
-
-    return True, None
-
-
-def validate_file_path(path: str) -> Tuple[bool, Optional[str]]:
-    """
-    Validate file path exists.
-    
-    Args:
-        path: File path
-        
-    Returns:
-        Tuple of (is_valid, error_message)
-    """
-    file_path = Path(path)
-    if not file_path.exists():
-        return False, f"File not found: {path}"
-    
-    if not file_path.is_file():
-        return False, f"Path is not a file: {path}"
-    
-    return True, None