Fix Last.fm import failures for large libraries with custom API keys and retry logic

[Copilot]

Last.fm import fails with HTTP 500 errors when users have large libraries (100k+ songs) due to rate limiting on the shared application API key.

Changes

Custom API Key Support

• Added optional API key field in Last.fm settings UI
• LastFMImport uses custom key if provided, falls back to default
• Key stored in settings under LastFM/api_key

// In CreateRequest()
const QString api_key = !api_key_.isEmpty() ? api_key_ : QLatin1String(LastFMScrobbler::kApiKey);
ParamList params = ParamList() << Param(u"api_key"_s, api_key) << ...;

Exponential Backoff Retry

• Retry HTTP 500/503 errors up to 5 times with exponential backoff (5s, 10s, 20s, 40s, 80s)
• Request structures track retry_count
• Helper methods: ShouldRetryRequest(), CalculateBackoffDelay(), LogRetryAttempt()
• Integer overflow protection in bit shift calculations

if (ShouldRetryRequest(result) && request.retry_count < kMaxRetries) {
  const int delay_ms = CalculateBackoffDelay(request.retry_count);
  QTimer::singleShot(delay_ms, this, [this, request]() {
    SendGetRecentTracksRequest(GetRecentTracksRequest(request.page, request.retry_count + 1));
  });
}

UI Changes

New field in Last.fm settings section with help text linking to API key creation page.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Last.fm Import does not work for large libraries</issue_title>
<issue_description>

https://github.com/strawberrymusicplayer/strawberry/blob/afc3effc9d6bcfec83f60e9b90bd55dbac14c672/src/scrobbler/lastfmscrobbler.cpp#L69C13-L69C28

A possible solution would be to allow the user to specify a last.fm api key, which they own, either manually (via an environment variable, config file or passed in argument) or via the ui.

Additionally, some more resilience (backoff logic) would allow large libraries to eventually successfully be fetched even with the shared application api key, even if it would be slower.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Last.fm Import does not work for large libraries #1936

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