fix(api): harden random-art retry loop

Three bugs in Random():

- The retry loop was unbounded. Only the NSFW path incremented attempt,
  so a run of DeviantArt errors span forever, hammering the API and
  risking an egress-IP ban.
- string(rand.Intn(999)) converts a rune, not a number: string(65) is
  "A", not "65". Searches were querying garbage. Use strconv.Itoa.
- rand.Intn panics on 0, so an empty result set crashed the handler.
  Skip empty results.

The exhausted-retries error now fires when the loop ends rather than
falling through to index an empty slice.

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