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>

cli/src/app.rs

@@ -421,7 +421,7 @@ impl App {
                     _ => None,
                 };
                 if let (Some(mt), Ok(bytes)) = (mt, std::fs::read(&path)) {
-                    self.images.push(ImageData { media_type: mt.into(), data: base64::engine::general_purpose::STANDARD.encode(bytes) });
+                    self.images.push(ImageData { blob: None, size: None, media_type: mt.into(), data: base64::engine::general_purpose::STANDARD.encode(bytes) });
                 }
             }
         }
@@ -840,7 +840,7 @@ impl App {
         for (cmd, args) in tries {
             if let Ok(out) = std::process::Command::new(cmd).args(*args).stderr(std::process::Stdio::null()).output() {
                 if out.status.success() && out.stdout.starts_with(b"\x89PNG") {
-                    self.images.push(ImageData { media_type: "image/png".into(), data: base64::engine::general_purpose::STANDARD.encode(&out.stdout) });
+                    self.images.push(ImageData { blob: None, size: None, media_type: "image/png".into(), data: base64::engine::general_purpose::STANDARD.encode(&out.stdout) });
                     self.status = format!("image {} attached", self.images.len());
                     return;
                 }

cli/src/tools.rs

@@ -408,7 +408,7 @@ pub async fn run(ctx: &Ctx, name: &str, input: &Value) -> Result<Output> {
                 return Ok(Output {
                     content: format!("Image {} ({} bytes)", rel(&ctx.cwd, &path), bytes.len()),
                     is_error: false,
-                    images: vec![ImageData { media_type: mt.into(), data: base64::engine::general_purpose::STANDARD.encode(&bytes) }],
+                    images: vec![ImageData { blob: None, size: None, media_type: mt.into(), data: base64::engine::general_purpose::STANDARD.encode(&bytes) }],
                     diff: None,
                 });
             }