Isolate provider search failures instead of failing the whole request

[lawrencejames24]

Summary

• MetadataService.searchSeriesMetadata launched each provider's search concurrently but awaited them without catching per-provider errors, so one failing provider (e.g. a 403 from a scraping-blocked source) would throw and discard results from every other provider that had already succeeded — surfacing as a 500 to callers even though most providers returned fine.
• Each provider's result is now caught individually and logged on failure, contributing only its own results to the aggregate instead of aborting the whole search.

Fixes #216 (at least for the manual/plain search path; the job-based auto-match flow has its own separate error handling in the same file that could use a similar treatment if useful).

Test plan

• ./gradlew :komf-mediaserver:compileKotlinJvm builds clean
• Reproduced the original failure mode locally: one provider (BookWalker) returning 403 while others (Viz, AniList, YenPress, MangaUpdates) returned 200 caused the whole search to fail before this change