CheckRedirect loads the stored rules and hands them to
checkRedirectRules, which needs no database, so the loop and chain rules
can be unit-tested. No behaviour change.
- rule set: exact sources by key, patterns by literal prefix, regex by id
(stdlib RE2, compiled once per source); ignore case, trailing slash,
query ignore/pass, a source with ? matches its query exactly
- targets: captures escaped, a relative target never leaves the site, an
absolute one keeps its host
- SaveRedirect validates, simulates the rule in place (exact, pattern and
regex probes, 10 hops) and refuses a loop; chains answer the notice
- resolve follows the chain to its final target, 410 answers gone, a
runtime cycle answers not_found and is logged once
