Refactor: Improve error handling and robustness

[shimpeiws]

Summary

Several areas of the codebase can benefit from improved type safety and robustness.

Items

1. Type-safe `_retry_request`

In genius/client.py, _retry_request accepts request_fn as Any and returns Any. This should use generics:

```python
from typing import TypeVar, Callable
T = TypeVar("T")

def _retry_request(self, request_fn: Callable[[], T], retries: int | None = None) -> T:
```

2. Cache race condition handling

cache.py uses file I/O for get_lyrics() and store_lyrics(). If lyrics fetching is parallelized, concurrent access to the same song's cache file could cause race conditions.

Proposed fix: Use atomic writes (write to temp file then rename) or adopt the filelock library.

3. Simplify `_get_cache_path` hashing

cache.py uses hashlib.md5(str(song_id).encode()).hexdigest()[:2] for directory distribution, but since song_id is an integer, a simple song_id % 256 would suffice without the MD5 overhead.

Affected Files

• src/barscan/genius/client.py
• src/barscan/genius/cache.py