Retry Spotify 429s with backoff before surfacing a rate-limit message

[chiptus]

Parent

#405

What to build

When the search-artist-links edge function's Spotify path gets rate-limited (429), it retries once or twice with backoff honoring the response's Retry-After header instead of failing immediately. If still rate-limited after retries, the client receives a distinguishable rate-limit error (with wait time) instead of the current generic Spotify search failed (${status}) message, so the Link Wizard can show "rate limited, try again in Ns."

Acceptance criteria

• A retry-with-backoff helper: succeeds immediately on a non-429 or first-try success (no retry); on 429, retries honoring Retry-After; after exhausting retries, returns a distinguishable rate-limit result carrying the wait time.
• Non-429 errors are not retried and fail immediately as today.
• The Link Wizard UI shows a clear "rate limited, try again in Ns" message when this error is surfaced, instead of the generic failure message.
• SoundCloud's existing 429 handling is unchanged (out of scope here).

Blocked by

None (can start immediately)