Provides `Singleton` and `ThreadSafe` metaclasses for enforcing single-instance patterns across the codebase. Used by Config, EventBus, services, and repositories. Thread-safe with double-checked locking. Easily extractable for any Python project needing thread-safe singletons.

Implements an `EventBus` singleton with `subscribe`/`emit` methods, supporting both sync and async listeners. Events are typed dataclasses. Mutation between listeners and event cancellation are supported. Decouples components (e.g., download state changes, config updates) and is reusable across async systems.

A thread-safe background worker that can execute sync and async functions with a configurable queue and shutdown coordination. Handles exceptions and provides a callback mechanism. Extracted this pattern is useful for any async application needing to offload blocking operations.

Provides a `@route()` decorator that auto-registers aiohttp view handlers, supports HTTP methods, path, and permission levels (public, auth_only, cookie_only). Injects services via inspection. This pattern cleanly separates route registration from handler logic and could be reused as a lightweight routing framework.

Contains `rename_file` and `move_file` functions that automatically handle sidecar files (subtitles, thumbnails, metadata) to keep them in sync. Also implements `safe_path`, `merge_dict` (with depth/size limits and dunder key filtering), and `timed_lru_cache` decorator. These utility functions are well-tested and reusable outside the project.