fix(link): prevent unbounded requestdl API calls on persistent CDN 404

[TwistedRat]

Problem

Two commits together cause unbounded requestdl API calls that can trigger TorBox's cooldown_until mechanism:

1. 4a7777c (already in beta via this PR) — CDN 404 was changed from CategoryPermanent to CategoryRefetchable so that expired CDN URLs are re-fetched rather than cached as permanent failures. This is the correct behaviour for a stale URL, but it has a side-effect:

2. The loop: invalidateAndRefetch deletes the link from the account-level cache and calls requestdl to get a fresh CDN URL. The new URL is returned but not stored in s.validated. On the next rclone/Plex poll (every few seconds), s.validated has no entry → HEAD check → 404 → another invalidateAndRefetch → another requestdl call. This repeats on every poll for as long as the file returns 404 — which can be minutes while TorBox is still processing a newly-added torrent.

The result is hundreds of requestdl calls per file per hour. TorBox's API terms treat this as automated abuse and apply an extended cooldown_until freeze on the API key.

Fix

pkg/manager/link/errors.go — no change from what landed in beta; 404 stays CategoryRefetchable.

pkg/manager/link/service.go — adds a per-file refetch cooldown map:

• After any 404-triggered invalidateAndRefetch, the file key (infohash:filename) is recorded with a 5-minute expiry.
• Subsequent polls within that window return the cached error immediately without calling requestdl.
• On a successful HEAD validation the cooldown is cleared, so a file that becomes available within the window is served promptly.

The cooldown is intentionally short (5 min) — long enough to stop the flood, short enough that a newly-processed torrent is retried automatically.

Also included

• Exponential backoff for transient errors (429, 502, 503, 504) during link validation — prevents a single rate-limit spike from being cached as a permanent failure.
• 502/504 added as CategoryRetryable (previously fell through to CategoryPermanent).
• Retryable errors are no longer stored in s.validated — a 502 that clears after 10 s would otherwise block a file for the lifetime of the process.

Reproducer

1. Add a torrent to TorBox that takes > 30 s to process.
2. Radarr/Sonarr immediately requests the file via the rclone FUSE mount.
3. Without this fix: requestdl is called every few seconds until TorBox finishes processing.
4. With this fix: one requestdl call on first 404, then silence for 5 min, then one retry.

Testing

Confirmed against a live TorBox Pro account. The cooldown_until field in /v1/api/user/me was set by the pre-fix behaviour and expired after the fix was deployed.