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/runner.rs

@@ -291,7 +291,7 @@ pub async fn run_session(app: Arc<App>, session_id: &str, cancel: CancellationTo
 
         let owner = session.owner_id.as_deref().and_then(|id| app.store.user(id).ok().flatten());
         let tools = available_tools(&app, &agent, owner.as_ref());
-        let history = active_history(app.store.messages(session_id)?);
+        let history = active_history(app.store.messages_for_model(session_id)?);
         let req = ChatRequest {
             system: prompts::with_chat_instructions(prompts::system_prompt(&app, &agent, &root, &cwd, &model_spec, &session.permission_mode, owner.as_ref()), &session.instructions),
             messages: history,
@@ -1126,7 +1126,7 @@ async fn review_turn(app: &Arc<App>, session: &Session, agent: &prompts::Agent,
     model.effort = Some("medium".into());
     app.emit(Event::Waiting { session_id: session.id.clone(), reason: Some(format!("review:{round}/{rounds}")) });
     let result = async {
-        let history = active_history(app.store.messages(&session.id)?);
+        let history = active_history(app.store.messages_for_model(&session.id)?);
         // The user's request: the last user message with text that is not feedback/tool output.
         let req_idx = history
             .iter()
@@ -1251,7 +1251,7 @@ pub async fn compact(app: Arc<App>, session_id: &str, cancel: CancellationToken,
 
 async fn compact_inner(app: Arc<App>, session_id: &str, cancel: CancellationToken, auto: Option<(u64, u64)>) -> Result<()> {
     let session = app.session(session_id)?;
-    let history = active_history(app.store.messages(session_id)?);
+    let history = active_history(app.store.messages_for_model(session_id)?);
     if history.is_empty() {
         return Ok(());
     }