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

packages/db/src/client.ts

@@ -12,7 +12,15 @@ export interface DbHandle {
 
 export function createDb(url?: string): DbHandle {
   const connectionString = url ?? process.env['DATABASE_URL'] ?? DEFAULT_DATABASE_URL;
-  const sql = postgres(connectionString);
+  const sql = postgres(connectionString, {
+    // Pool sizing: allow up to 20 concurrent connections per gateway instance.
+    // Each NestJS module (brain, preferences, memory, coord) shares this pool.
+    max: Number(process.env['DB_POOL_MAX'] ?? 20),
+    // Recycle idle connections after 30 s to avoid stale TCP state.
+    idle_timeout: Number(process.env['DB_IDLE_TIMEOUT'] ?? 30),
+    // Fail fast (5 s) on connection problems rather than hanging indefinitely.
+    connect_timeout: Number(process.env['DB_CONNECT_TIMEOUT'] ?? 5),
+  });
   const db = drizzle(sql, { schema });
   return { db, close: () => sql.end() };
 }