Chunk the silent-payment catch-up scan

An imported mainnet wallet's first scan pulled the indexer's whole
window (~336k tweaks, 25MB) and posted it to the worker as ONE message:
it blew the 30s worker timeout, fell back to the inline main-thread
loop (minutes of jank) while the worker kept grinding the orphaned
request — the runaway CPU/memory users saw. The catch-up now processes
100-block chunks: each worker call is seconds, the scan watermark
checkpoints after every chunk (an interrupted catch-up resumes instead
of restarting from zero), buffers free as it goes, and the UI yields
between chunks. Verified against the live indexer: full catch-up in
~5min of background work, main heap bounded (~80MB peak), no timeouts.

Co-Authored-By: Claude Fable 5 <[email protected]>

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