Refactor searchMissingBatch to schedule each source independently via a
per-source promise chain (scheduleSource) instead of processing all targets
in a single serial loop. This keeps source ordering intact while allowing
different sources to advance in parallel across models.
Also update searchOnline to accept scheduleSource and emit a pending UI
state for queued searches, split pending/running source colors in CSS, and
add tests covering independent batch source progression plus error/cancel
survival.