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/store.rs

@@ -1,6 +1,6 @@
 //! SQLite persistence for sessions and messages.
 
-use crate::types::{Message, Part, Session, now_ms};
+use crate::types::{ImageData, Message, Part, Session, now_ms};
 use serde::Serialize;
 use serde_json::{Value, json};
 use anyhow::Result;
@@ -90,6 +90,13 @@ impl Store {
                 created_at INTEGER NOT NULL
              );
              CREATE INDEX IF NOT EXISTS images_user ON images(user_id, created_at);
+             -- Images of messages, referenced by hash from the message JSON (see blobs.rs).
+             CREATE TABLE IF NOT EXISTS blobs (
+                hash TEXT PRIMARY KEY,
+                media_type TEXT NOT NULL,
+                data BLOB NOT NULL,
+                created_at INTEGER NOT NULL
+             );
              CREATE TABLE IF NOT EXISTS queue (
                 id TEXT PRIMARY KEY,
                 session_id TEXT NOT NULL REFERENCES sessions(id) ON DELETE CASCADE,
@@ -274,12 +281,21 @@ impl Store {
     }
 
     pub fn add_message(&self, session_id: &str, m: &Message) -> Result<()> {
+        // Images go to the blob table (encoded before taking the lock).
+        let mut m = m.clone();
+        let blobs = crate::blobs::externalize(&mut m);
         let conn = self.conn.lock().unwrap();
+        for b in &blobs {
+            conn.execute(
+                "INSERT OR IGNORE INTO blobs (hash, media_type, data, created_at) VALUES (?1, ?2, ?3, ?4)",
+                params![b.hash, b.media_type, b.bytes, now_ms()],
+            )?;
+        }
         conn.execute(
             "INSERT INTO messages (id, session_id, seq, data) VALUES (?1, ?2,
                 (SELECT COALESCE(MAX(seq), 0) + 1 FROM messages WHERE session_id = ?2), ?3)
              ON CONFLICT(id) DO UPDATE SET data = excluded.data",
-            params![m.id, session_id, serde_json::to_string(m)?],
+            params![m.id, session_id, serde_json::to_string(&m)?],
         )?;
         conn.execute(
             "UPDATE sessions SET updated_at = ?2 WHERE id = ?1",
@@ -300,6 +316,103 @@ impl Store {
         Ok(out)
     }
 
+    /// Messages with the bytes of their images filled in – for model requests and exports.
+    pub fn messages_for_model(&self, session_id: &str) -> Result<Vec<Message>> {
+        let mut out = self.messages(session_id)?;
+        let conn = self.conn.lock().unwrap();
+        let mut cache: std::collections::HashMap<String, String> = std::collections::HashMap::new();
+        for m in &mut out {
+            crate::blobs::for_each_image(m, |img| fill_image(&conn, &mut cache, img));
+        }
+        Ok(out)
+    }
+
+    /// Fill in the bytes of one image reference (no-op for inline images).
+    pub fn resolve_image(&self, img: &ImageData) -> ImageData {
+        let mut img = img.clone();
+        let conn = self.conn.lock().unwrap();
+        fill_image(&conn, &mut std::collections::HashMap::new(), &mut img);
+        img
+    }
+
+    /// One stored image: (media type, bytes).
+    pub fn blob(&self, hash: &str) -> Result<Option<(String, Vec<u8>)>> {
+        let conn = self.conn.lock().unwrap();
+        Ok(conn
+            .query_row("SELECT media_type, data FROM blobs WHERE hash = ?1", [hash], |r| Ok((r.get(0)?, r.get(1)?)))
+            .optional()?)
+    }
+
+    /// Move inline images of older messages to the blob table (once, after the update
+    /// that introduced it). Does not touch `updated_at` of the sessions.
+    pub fn migrate_inline_images(&self) -> Result<usize> {
+        let ids: Vec<String> = {
+            let conn = self.conn.lock().unwrap();
+            let mut stmt = conn.prepare("SELECT id FROM messages WHERE data LIKE '%\"media_type\"%\"data\":\"%'")?;
+            let rows = stmt.query_map([], |r| r.get(0))?.collect::<rusqlite::Result<Vec<String>>>()?;
+            rows
+        };
+        let mut moved = 0;
+        for id in ids {
+            let data: Option<String> = {
+                let conn = self.conn.lock().unwrap();
+                conn.query_row("SELECT data FROM messages WHERE id = ?1", [&id], |r| r.get(0)).optional()?
+            };
+            let Some(data) = data else { continue };
+            let Ok(mut m) = serde_json::from_str::<Message>(&data) else { continue };
+            let blobs = crate::blobs::externalize(&mut m);
+            if blobs.is_empty() {
+                continue;
+            }
+            let conn = self.conn.lock().unwrap();
+            for b in &blobs {
+                conn.execute(
+                    "INSERT OR IGNORE INTO blobs (hash, media_type, data, created_at) VALUES (?1, ?2, ?3, ?4)",
+                    params![b.hash, b.media_type, b.bytes, now_ms()],
+                )?;
+            }
+            conn.execute("UPDATE messages SET data = ?2 WHERE id = ?1", params![id, serde_json::to_string(&m)?])?;
+            moved += blobs.len();
+        }
+        Ok(moved)
+    }
+
+    /// Delete images no message refers to any more (deleted or truncated sessions).
+    pub fn gc_blobs(&self) -> Result<usize> {
+        let conn = self.conn.lock().unwrap();
+        let mut used = std::collections::HashSet::new();
+        let mut stmt = conn.prepare("SELECT data FROM messages WHERE data LIKE '%\"blob\":\"%'")?;
+        let mut rows = stmt.query([])?;
+        while let Some(row) = rows.next()? {
+            let data: String = row.get(0)?;
+            for (i, _) in data.match_indices("\"blob\":\"") {
+                if let Some(h) = data.get(i + 8..i + 72) {
+                    used.insert(h.to_string());
+                }
+            }
+        }
+        drop(rows);
+        drop(stmt);
+        let all: Vec<String> = conn.prepare("SELECT hash FROM blobs")?.query_map([], |r| r.get(0))?.collect::<rusqlite::Result<_>>()?;
+        let mut removed = 0;
+        for h in all.iter().filter(|h| !used.contains(*h)) {
+            removed += conn.execute("DELETE FROM blobs WHERE hash = ?1", [h])?;
+        }
+        Ok(removed)
+    }
+
+    /// Give the space of deleted data back to the file system when it is worth it.
+    pub fn vacuum_if_worthwhile(&self) -> Result<bool> {
+        let conn = self.conn.lock().unwrap();
+        let free: i64 = conn.query_row("PRAGMA freelist_count", [], |r| r.get(0))?;
+        let page: i64 = conn.query_row("PRAGMA page_size", [], |r| r.get(0))?;
+        if free * page < 32 * 1024 * 1024 {
+            return Ok(false);
+        }
+        conn.execute_batch("VACUUM")?;
+        Ok(true)
+    }
+
     /// Remove all messages after (and including) the given one – used for "edit & resend".
     pub fn truncate_from(&self, session_id: &str, message_id: &str) -> Result<()> {
         let conn = self.conn.lock().unwrap();
@@ -312,6 +425,20 @@ impl Store {
     }
 }
 
+/// Put the bytes of a stored image into `img.data`.
+fn fill_image(conn: &Connection, cache: &mut std::collections::HashMap<String, String>, img: &mut ImageData) {
+    use base64::Engine;
+    let Some(hash) = img.blob.clone().filter(|_| img.data.is_empty()) else { return };
+    if !cache.contains_key(&hash) {
+        let Ok(bytes) = conn.query_row("SELECT data FROM blobs WHERE hash = ?1", [&hash], |r| r.get::<_, Vec<u8>>(0)) else {
+            tracing::warn!("image {hash} is missing in the store");
+            return;
+        };
+        cache.insert(hash.clone(), base64::engine::general_purpose::STANDARD.encode(bytes));
+    }
+    img.data = cache[&hash].clone();
+}
+
 /// One model request, as recorded for statistics.
 #[derive(Debug, Clone, Serialize)]
 pub struct UsageRecord {