fix: Improve Whisper model status verification and PyInstaller builds

- Add robust model cache verification (check model.bin + config.json)
- Add new status messages: model_cached, incomplete_cache, model_error
- Forward model status events to frontend for better UI feedback
- Add clean_build_cache() to remove stale spec files before build
- Add --clean flag to PyInstaller commands
- Change sidecar from --onefile to --onedir for faster startup
- Add missing hidden imports: onnxruntime, wave, huggingface_hub.utils

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/build.py

@@ -10,14 +10,40 @@ import os
 import shutil
 
 
+def clean_build_cache(script_dir):
+    """Clean old build artifacts that may cause stale spec file issues."""
+    dirs_to_clean = [
+        os.path.join(script_dir, "build"),
+        os.path.join(script_dir, "__pycache__"),
+    ]
+    files_to_clean = [
+        os.path.join(script_dir, "build", "backend.spec"),
+    ]
+
+    for f in files_to_clean:
+        if os.path.exists(f):
+            print(f"Removing old spec file: {f}")
+            os.remove(f)
+
+    for d in dirs_to_clean:
+        pycache = os.path.join(d)
+        if os.path.exists(pycache) and "__pycache__" in pycache:
+            print(f"Removing cache: {pycache}")
+            shutil.rmtree(pycache)
+
+
 def build():
     """Build the backend executable."""
     script_dir = os.path.dirname(os.path.abspath(__file__))
 
+    # Clean old build cache to avoid stale spec file issues
+    clean_build_cache(script_dir)
+
     # PyInstaller command with --onedir for faster startup
     cmd = [
         sys.executable, "-m", "PyInstaller",
         "--onedir",
+        "--clean",  # Clean PyInstaller cache before building
         "--name", "backend",
         "--distpath", "dist",
         "--workpath", "build",
@@ -39,9 +65,11 @@ def build():
         "--hidden-import", "starlette",
         "--hidden-import", "pydantic",
         "--hidden-import", "pydantic_core",
-        # Database
+        # Database - MySQL
         "--hidden-import", "mysql.connector",
         "--hidden-import", "mysql.connector.pooling",
+        # Database - SQLite (built-in, but ensure it's included)
+        "--hidden-import", "sqlite3",
         # HTTP client
         "--hidden-import", "httpx",
         "--hidden-import", "httpcore",
@@ -56,7 +84,9 @@ def build():
         "--hidden-import", "python_multipart",
         # Environment loading
         "--hidden-import", "dotenv",
-        # Application modules
+        # Timezone data
+        "--hidden-import", "tzdata",
+        # Application modules - only include modules that exist
         "--hidden-import", "app",
         "--hidden-import", "app.main",
         "--hidden-import", "app.config",