address CodeRabbit review: fix 4 critical + 12 major issues

Critical:
- Remove dead backend.utils.validation PyInstaller hidden import
- Fix story_items table rebuild to preserve track/trim/version columns
- Guard cache migration against same source/destination path
- Fix regeneration audio overwrite (use random uuid suffix per take)

Major:
- Engine selector: validate language on Qwen switch, clear stale modelSize
- Sync language validation regex between profile create and generate (22 langs)
- Guard CUDA download against duplicate concurrent requests
- Only set model_size for engines that support multiple sizes
- Fix 404 swallowed by generic except in history export
- Validate audio_path before FileResponse in export-audio
- Transcription: stream uploads in 1MB chunks, use robust cache check,
  call complete_download() on Whisper download success
- Set clean version as default when effects chain validation fails
- Return explicit error when Windows port occupied by non-voicebox process

backend/database/migrations.py

@@ -92,14 +92,20 @@ def _migrate_story_items(engine, inspector, tables: set[str]) -> None:
                     story_id VARCHAR NOT NULL,
                     generation_id VARCHAR NOT NULL,
                     start_time_ms INTEGER NOT NULL DEFAULT 0,
+                    track INTEGER NOT NULL DEFAULT 0,
+                    trim_start_ms INTEGER NOT NULL DEFAULT 0,
+                    trim_end_ms INTEGER NOT NULL DEFAULT 0,
+                    version_id VARCHAR,
                     created_at DATETIME,
                     FOREIGN KEY (story_id) REFERENCES stories(id),
                     FOREIGN KEY (generation_id) REFERENCES generations(id)
                 )
             """))
             conn.execute(text("""
-                INSERT INTO story_items_new (id, story_id, generation_id, start_time_ms, created_at)
-                SELECT id, story_id, generation_id, start_time_ms, created_at FROM story_items
+                INSERT INTO story_items_new (id, story_id, generation_id, start_time_ms, track, trim_start_ms, trim_end_ms, version_id, created_at)
+                SELECT id, story_id, generation_id, start_time_ms,
+                    COALESCE(track, 0), COALESCE(trim_start_ms, 0), COALESCE(trim_end_ms, 0), version_id, created_at
+                FROM story_items
             """))
             conn.execute(text("DROP TABLE story_items"))
             conn.execute(text("ALTER TABLE story_items_new RENAME TO story_items"))