Store chat images separately; JPEG for large PNGs; gzip/brotli responses

Images of messages (pasted images, screenshots, generated images) were base64
inside the message JSON, so opening a chat downloaded all of them at once
(66 MB for a chat with 120 screenshots). They now live in a `blobs` table,
addressed by SHA-256; messages keep a reference (blob, size). The UI loads
them from /api/blobs/<hash> lazily with an immutable cache header, the runner
fills the bytes in before model requests, JSON exports stay self-contained.

- Store::add_message moves inline images out; large opaque PNGs become JPEG
  (q85) when smaller. Older chats are converted once at start-up in the
  background, unused images are removed (also after deleting a chat) and the
  database is vacuumed when that frees 32 MB or more.
- Responses are compressed with gzip/brotli (not images, SSE streams or
  binary downloads).

Example (copy of production): chat load 66 MB -> 1.5 MB (370 KB brotli),
database 90 MB -> 27 MB.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/src/mcp.rs

@@ -212,6 +212,8 @@ impl McpServer {
             match c["type"].as_str().unwrap_or_default() {
                 "text" => texts.push(c["text"].as_str().unwrap_or_default().to_string()),
                 "image" => out.images.push(ImageData {
+                    blob: None,
+                    size: None,
                     media_type: c["mimeType"].as_str().unwrap_or("image/png").into(),
                     data: c["data"].as_str().unwrap_or_default().into(),
                 }),