Feed dates stick to publish time; GitHub fetches survive rate limits

Fixed

• Atom feed releases now record the date they were first published instead of the date the entry was last edited — an edit no longer silently moves a release forward in time.
• GitHub-sourced changelogs now keep releases collected before a rate limit — a 403 rate-limit response is now treated the same as 429, returning partial results instead of aborting the fetch.
• Sign-in no longer fails during transient KV storage errors in rate-limit tracking; the error is logged and the request continues.
• Updating a release URL via the API now returns 409 with the conflicting release ID when the URL is already held by another release in the same source, instead of 500.
• Transient database errors during a health probe now return a retryable 503 with Retry-After instead of the misleading "run migrations first" message.