NFR-1: Latency

• Cached descriptions: instant (≤ 50ms from cache read to client response)
• Fresh descriptions: ≤ 15 seconds end-to-end (image retrieval + API call + response parsing)
• The client must not block waiting for descriptions — messages load immediately, and description bubbles appear asynchronously when ready
• If a description is pending, show a placeholder bubble: “[Describing image…]” with a subtle animation (e.g., pulsing dot)