feat(gamification): enforce the per-key XP rate limit on the award path (#12390)

validateScoreChange() — the documented 1000 XP/min per-API-key limit plus the velocity anomaly check — was exported but never called, so the award path applied every XP delta unconditionally. It now runs before addXp; a rejected award is logged at warn level and skipped, and the fire-and-forget path never throws.

The second finding is the one that made the first invisible: getRecentXp's window query was inert. created_at is stored by the table default as YYYY-MM-DD HH:MM:SS and was compared lexically against a JS ISO string, so same-day rows never matched and the limit could not have tripped even if it had been wired. The window start is now computed in SQLite, matching the style computeZScore already used.

Validated in a combined worktree with all 25 PRs of this batch boarded together: typecheck:core clean, 443/443 node-runner tests plus 14/14 vitest across every test file the batch touches, and check-changelog-integrity, check:cycles (418 files), check:provider-consistency (272 REGISTRY entries, 355 canonical providers), check:docs-counts, check:docs-sync (42 locales) and check-file-size all green.

Thanks @pacocartones.