fix(fetch): jitter the retry backoff and honor a429's Retry-After

Two gaps in fetch_with_attempts: the sleep was the bare 1 << attempt, so every worker that failed together (a source coming back, a shared proxy blip) also recovered on the same tick and re-stamped the source; and a429 arrived as an anonymous Transient, its Retry-After header — the one place a source tells you exactly how long it wants silence — dropped on the floor.

retry_wait(attempt, roll, err) is the pure decision: doubling base plus a random slice of itself ([base, 2x base)), floored at a named Retry-After. status_error now takes the response, reads the seconds-form header and returns the new FetchError::RateLimited { site, retry_after_secs } (HTTP-date parses to None and stays transient); the pure table moved to classify_status so tests still build it from bare status codes. The delay is capped at MAX_RETRY_AFTER_SECS = 60 — the header is server-supplied and must not park one of the eight fetch slots.

Everywhere a Transient meant 'retryable' the new variant joins: the Site trait default, pixiv's override (plus its ugoira-zip mapping), bsky's HLS second attempt, the download classifier, and the user-facing message arm. Tests pin the429 rows (with and without the header, cap included) and retry_wait's math; AGENTS' retries bullet and variant list follow.

crates/xmedia-bot/src/handlers/urls.rs

@@ -507,7 +507,7 @@ fn fetch_error_message(err: &x_media::site::FetchError) -> String {
         FetchError::Disabled { site } => {
             format!("{} support is disabled on this bot.", site_title(site))
         }
-        FetchError::Transient(_) | FetchError::Http(_) => {
+        FetchError::RateLimited { .. } | FetchError::Transient(_) | FetchError::Http(_) => {
             "The source site is unavailable right now (tried 3 times). Try again later.".to_string()
         }
         FetchError::MediaPrep(_) => concat!(