Remove the file mtime check from the API debounce tier in
get_sheet_modified_time(). Previously, the debounce was defeated when
CACHE_TTL_SECONDS differed from CACHE_API_CHECK_TTL_SECONDS because
the file age check would fail even though the API was checked recently.
Also fix cache key mappings (attendance_juniors sheet ID,
payments_transactions rename) and add tmp/ to .gitignore.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>