Adds an opt-in exhaustive mode to the quarantine-retry path. Default
behaviour is unchanged: a single global cap (MAX_QUARANTINE_RETRIES=5).
When post_processing.retry_exhaustive is on, each source gets its OWN
retry budget sized as query_count x retries_per_query. Soulseek peers
collapse to one 'soulseek' bucket; streaming plugins keep their name.
The worker now records query_count on the task; the budget scales with
the track's real query count. Loop protection is threefold: per-source
cap, used_sources exhaustion (the natural terminator), and an absolute
ceiling (MAX_TOTAL_QUARANTINE_RETRIES=100).
New settings (config + WebUI): retry_next_candidate_on_mismatch (master),
retry_exhaustive, retries_per_query (default 5).
Tests: 6 new cases covering per-source budgeting, source separation,
Soulseek-peer bucketing, query_count default, and the absolute ceiling.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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