FR-4: Description Caching

• Descriptions are cached locally on the Mac (where the relay runs) to avoid re-describing the same image
• Cache key: attachment GUID (from BlueBubbles)
• Cache storage: a JSON file at ~/.cache/aquaping/image_descriptions.json or a SQLite database
• Cache entry format: {guid: {description: "...", timestamp: "...", model: "minimax"}}
• Cache is read before every API call; if a GUID exists, the cached description is returned immediately
• Cache has no expiration (images don’t change)
• Cache is written after every successful API call