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/brain/src/conversations.ts

@@ -1,4 +1,9 @@
-import { eq, type Db, conversations, messages } from '@mosaic/db';
+import { eq, asc, desc, type Db, conversations, messages } from '@mosaic/db';
+
+/** Maximum number of conversations returned per list query. */
+const MAX_CONVERSATIONS = 200;
+/** Maximum number of messages returned per conversation history query. */
+const MAX_MESSAGES = 500;
 
 export type Conversation = typeof conversations.$inferSelect;
 export type NewConversation = typeof conversations.$inferInsert;
@@ -8,7 +13,12 @@ export type NewMessage = typeof messages.$inferInsert;
 export function createConversationsRepo(db: Db) {
   return {
     async findAll(userId: string): Promise<Conversation[]> {
-      return db.select().from(conversations).where(eq(conversations.userId, userId));
+      return db
+        .select()
+        .from(conversations)
+        .where(eq(conversations.userId, userId))
+        .orderBy(desc(conversations.updatedAt))
+        .limit(MAX_CONVERSATIONS);
     },
 
     async findById(id: string): Promise<Conversation | undefined> {
@@ -36,7 +46,12 @@ export function createConversationsRepo(db: Db) {
     },
 
     async findMessages(conversationId: string): Promise<Message[]> {
-      return db.select().from(messages).where(eq(messages.conversationId, conversationId));
+      return db
+        .select()
+        .from(messages)
+        .where(eq(messages.conversationId, conversationId))
+        .orderBy(asc(messages.createdAt))
+        .limit(MAX_MESSAGES);
     },
 
     async addMessage(data: NewMessage): Promise<Message> {