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/images.rs

@@ -258,7 +258,7 @@ fn preview_for_model(path: &Path) -> Option<ImageData> {
     let img = img.resize(1024, 1024, image::imageops::FilterType::Triangle);
     let mut buf = Vec::new();
     img.write_to(&mut std::io::Cursor::new(&mut buf), image::ImageFormat::Png).ok()?;
-    Some(ImageData { media_type: "image/png".into(), data: base64::engine::general_purpose::STANDARD.encode(buf) })
+    Some(ImageData { blob: None, size: None, media_type: "image/png".into(), data: base64::engine::general_purpose::STANDARD.encode(buf) })
 }
 
 pub struct GenerateImage;