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/preferences/preferences.service.ts

@@ -1,5 +1,5 @@
 import { Inject, Injectable, Logger } from '@nestjs/common';
-import { eq, and, type Db, preferences as preferencesTable } from '@mosaic/db';
+import { eq, and, sql, type Db, preferences as preferencesTable } from '@mosaic/db';
 import { DB } from '../database/database.module.js';
 
 export const PLATFORM_DEFAULTS: Record<string, unknown> = {
@@ -88,25 +88,24 @@ export class PreferencesService {
   }
 
   private async upsertPref(userId: string, key: string, value: unknown): Promise<void> {
-    const existing = await this.db
-      .select({ id: preferencesTable.id })
-      .from(preferencesTable)
-      .where(and(eq(preferencesTable.userId, userId), eq(preferencesTable.key, key)))
-      .limit(1);
-
-    if (existing.length > 0) {
-      await this.db
-        .update(preferencesTable)
-        .set({ value: value as never, updatedAt: new Date() })
-        .where(and(eq(preferencesTable.userId, userId), eq(preferencesTable.key, key)));
-    } else {
-      await this.db.insert(preferencesTable).values({
+    // Single-round-trip upsert using INSERT … ON CONFLICT DO UPDATE.
+    // Previously this was two queries (SELECT + INSERT/UPDATE), which doubled
+    // the DB round-trips and introduced a TOCTOU window under concurrent writes.
+    await this.db
+      .insert(preferencesTable)
+      .values({
         userId,
         key,
         value: value as never,
         mutable: true,
+      })
+      .onConflictDoUpdate({
+        target: [preferencesTable.userId, preferencesTable.key],
+        set: {
+          value: sql`excluded.value`,
+          updatedAt: sql`now()`,
+        },
       });
-    }
     this.logger.debug(`Upserted preference "${key}" for user ${userId}`);
   }