fix(verify): treat a rate-limit answer as transient, not as a dead link

[Seungpyo1007]

Problem

check-urls treats HTTP 429 as a liveness verdict and caches it for the full 30-day TTL.

Current cache contents by host:

host	result
gsmarena.com	3,998 http-429, 44 http-200
en.wikipedia.org	68 alive / 6 404
techpowerup.com	61 http-403, 25 alive

Probed directly, every sampled 429'd GSMArena URL answers 200, even with no pacing at all:

--- pacing 0.0s, GET ---
  200 1.5s  blackview_bv5200_pro-12056.php
  200 1.1s  blackview_bv5300_pro-12410.php
  200 1.0s  blackview_bv8900-12372.php

So ~4,000 records are unpromotable not because their sources are bad, but because one run asked too fast and the refusal was written down as a fact. There is already a 1 req/s per-host limiter — but once the host starts refusing, every remaining URL on it cascades into 429 at that same pace.

Change

• Retry 429/503 up to 3 times, honouring Retry-After when present (clamped to 15s).
• Back off per host: a host that pushes back gets its interval multiplied for the rest of the run, so the cascade stops.
• Never cache a rate limit: a still-throttled URL isn't written, and 429/503 entries already in the cache are ignored on load — which heals the 3,998 poisoned entries without a manual purge.

Liveness classification is otherwise unchanged: 404, 403, soft-404 redirects and connection errors are still dead.

4 regression tests added (retry-then-succeed, persistent-throttle-is-transient, host slowdown, poisoned-cache-entry-is-not-a-hit); tests/verify is 50 passed.

Refs #1