crawlindex:bulk-fetch walks Common Crawl's CDX shard files directly on
data.commoncrawl.org (via cc-index.paths.gz), instead of the
index.commoncrawl.org query API — usable for bulk backfill/refresh or
as a fallback when that API is down. Shares the group/save pipeline
with crawlindex:fetch via a new SavesCommonCrawlCaptures trait.
Also fixes a real bug this surfaced under load: site_pages.urlkey was
a varchar(255), which is too narrow for real-world captured URLs
(long query strings on spam/malformed pages overflow it). Widened to
text and switched the uniqueness constraint to a sha1 hash column,
since MySQL can't fully index a TEXT column.
m105
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