perf: gateway + DB + frontend optimizations (P8-003)

- DB client: configure connection pool (max=20, idle_timeout=30s, connect_timeout=5s)
- DB schema: add missing indexes for auth sessions, accounts, conversations, agent_logs
- DB schema: promote preferences(user_id,key) to UNIQUE index for ON CONFLICT upsert
- Drizzle migration: 0003_p8003_perf_indexes.sql
- preferences.service: replace 2-query SELECT+INSERT/UPDATE with single-round-trip upsert
- conversations repo: add ORDER BY + LIMIT to findAll (200) and findMessages (500)
- session-gc.service: make onModuleInit fire-and-forget (removes cold-start TTFB block)
- next.config.ts: enable compress, productionBrowserSourceMaps:false, image avif/webp
- docs/PERFORMANCE.md: full profiling report and change impact notes

apps/gateway/src/gc/session-gc.service.ts

@@ -36,16 +36,24 @@ export class SessionGCService implements OnModuleInit {
     @Inject(LOG_SERVICE) private readonly logService: LogService,
   ) {}
 
-  async onModuleInit(): Promise<void> {
-    this.logger.log('Running full GC on cold start...');
-    const result = await this.fullCollect();
-    this.logger.log(
-      `Full GC complete: ${result.valkeyKeys} Valkey keys, ` +
-        `${result.logsDemoted} logs demoted, ` +
-        `${result.jobsPurged} jobs purged, ` +
-        `${result.tempFilesRemoved} temp dirs removed ` +
-        `(${result.duration}ms)`,
-    );
+  onModuleInit(): void {
+    // Fire-and-forget: run full GC asynchronously so it does not block the
+    // NestJS bootstrap chain.  Cold-start GC typically takes 100–500 ms
+    // depending on Valkey key count; deferring it removes that latency from
+    // the TTFB of the first HTTP request.
+    this.fullCollect()
+      .then((result) => {
+        this.logger.log(
+          `Full GC complete: ${result.valkeyKeys} Valkey keys, ` +
+            `${result.logsDemoted} logs demoted, ` +
+            `${result.jobsPurged} jobs purged, ` +
+            `${result.tempFilesRemoved} temp dirs removed ` +
+            `(${result.duration}ms)`,
+        );
+      })
+      .catch((err: unknown) => {
+        this.logger.error('Cold-start GC failed', err instanceof Error ? err.stack : String(err));
+      });
   }
 
   /**