Scrobble tracks to Last.fm using track.scrobble with proper API method signature (api_sig) authentication

Verified steps

1. Register an API application on Last.fm to get an API key and shared secret
2. Obtain a session key by completing Last.fm's desktop auth flow (get an auth token, have the user authorize it in a browser, then exchange the token for a session key)
3. For each scrobble call, build a plain-text signature: concatenate every request parameter (excluding `format`) as `key` + `value` pairs sorted alphabetically by key, append your shared secret at the end with no key name, then MD5-hash the resulting string to produce `api_sig`
4. Send a `POST` to the Last.fm API root with `method=track.scrobble`, the required array-style parameters `artist[i]`, `track[i]`, `timestamp[i]` (Unix time the track started playing) for each scrobble, plus your `api_key`, session key, and computed `api_sig`
5. Batch up to the documented per-call maximum (50 scrobbles) rather than issuing one request per track when backfilling history
6. Only scrobble a track once the user has played at least half of it or 4 minutes, whichever is shorter, per Last.fm's scrobbling guidelines