perf: speed up startup from ~15s to ~2s (#280)

Three high-impact changes eliminate most startup latency:

1. Enable wasmtime persistent compilation cache — call
   cache_config_load_default() so compiled native code is serialized to
   disk (~/.cache/wasmtime). Subsequent startups deserialize instead of
   recompiling, dropping the WASM phase from ~13s to <1s.

2. Cache compiled Component in PreparedModule — store the compiled
   wasmtime::component::Component directly instead of raw bytes.
   Eliminates ~2.6s recompilation on every first tool/channel execution.

3. Move blocking housekeeping to background tasks — embedding backfill
   (~1.3s of failing HTTP calls) and stale job cleanup are fire-and-forget
   work that no longer blocks the critical startup path.

Also: deduplicate Workspace creation in main.rs (two identical instances
reduced to one), and replace leftover println! in session validation with
tracing calls.

Co-authored-by: Claude Opus 4.6 <[email protected]>

src/app.rs

@@ -200,9 +200,13 @@ impl AppBuilder {
 
         self.session.attach_store(db.clone(), "default").await;
 
-        if let Err(e) = db.cleanup_stale_sandbox_jobs().await {
-            tracing::warn!("Failed to cleanup stale sandbox jobs: {}", e);
-        }
+        // Fire-and-forget housekeeping — no need to block startup.
+        let db_cleanup = db.clone();
+        tokio::spawn(async move {
+            if let Err(e) = db_cleanup.cleanup_stale_sandbox_jobs().await {
+                tracing::warn!("Failed to cleanup stale sandbox jobs: {}", e);
+            }
+        });
 
         self.db = Some(db);
         Ok(())
@@ -715,15 +719,18 @@ impl AppBuilder {
             }
 
             if embeddings.is_some() {
-                match ws.backfill_embeddings().await {
-                    Ok(count) if count > 0 => {
-                        tracing::info!("Backfilled embeddings for {} chunks", count);
+                let ws_bg = Arc::clone(ws);
+                tokio::spawn(async move {
+                    match ws_bg.backfill_embeddings().await {
+                        Ok(count) if count > 0 => {
+                            tracing::info!("Backfilled embeddings for {} chunks", count);
+                        }
+                        Ok(_) => {}
+                        Err(e) => {
+                            tracing::warn!("Failed to backfill embeddings: {}", e);
+                        }
                     }
-                    Ok(_) => {}
-                    Err(e) => {
-                        tracing::warn!("Failed to backfill embeddings: {}", e);
-                    }
-                }
+                });
             }
         }