Started PostgreSQL — data directory was uninitialized, ran initdb, set password, switched to md5 auth, all 24 migrations applied.

Hardened db.ts — added statement_timeout: 30000, idle_in_transaction_session_timeout: 10000, created transaction() helper (BEGIN/COMMIT/ROLLBACK).
Multi-step API routes wrapped in transactions — Events POST, Conversations POST use atomic blocks.
Fixed leads pagination — status filter pushed into SQL WHERE (no client-side filtering after LIMIT/OFFSET), added SELECT COUNT(*) for total.
Rewrote dashboard — SQL aggregations (COUNT + GROUP BY + date_trunc) replace loading all leads into memory.
Optimized conversations GET — merged duplicate correlated subqueries into single LEFT JOIN LATERAL.
Created migration 021_performance_indexes.sql — 8 missing indexes + set_session_user_context() function caching current_user_hierarchy_level() as session variable (avoids per-query RLS join).
Fixed build error — duplicate const other in conversations route. Also fixed a TypeScript never error in dashboard breakdown map.
Verified — npx tsc --noEmit clean, npm test (13 pass, 7 integration skip gracefully), npx next build succeeds.

src/app/api/conversations/route.ts

@@ -1,6 +1,6 @@
 import { NextRequest, NextResponse } from "next/server"
 import { getSessionUser } from "@/lib/auth"
-import { query } from "@/lib/db"
+import { query, transaction } from "@/lib/db"
 import { avatarSvgUrl } from "@/lib/avatar"
 
 export async function GET() {
@@ -18,13 +18,18 @@ export async function GET() {
           u.email AS other_user_email,
           u.phone AS other_user_phone,
           u.avatar_url AS other_user_avatar_url,
-          (SELECT content FROM messages WHERE conversation_id = c.id AND deleted_at IS NULL ORDER BY created_at DESC LIMIT 1) AS last_message,
-          (SELECT created_at FROM messages WHERE conversation_id = c.id AND deleted_at IS NULL ORDER BY created_at DESC LIMIT 1) AS last_message_time,
+          lm.last_message_data->>'content' AS last_message,
+          (lm.last_message_data->>'created_at')::timestamptz AS last_message_time,
          (SELECT count(*) FROM messages WHERE conversation_id = c.id AND sender_id != $1 AND created_at > COALESCE(cp_me.last_read_at, '1970-01-01')) AS unread
        FROM conversations c
        JOIN conversation_participants cp_me ON cp_me.conversation_id = c.id AND cp_me.user_id = $1
        JOIN conversation_participants cp ON cp.conversation_id = c.id
        JOIN users u ON u.id = cp.user_id AND u.id != $1
+       LEFT JOIN LATERAL (
+         SELECT jsonb_build_object('content', content, 'created_at', created_at) AS last_message_data
+         FROM messages WHERE conversation_id = c.id AND deleted_at IS NULL
+         ORDER BY created_at DESC LIMIT 1
+       ) lm ON true
        WHERE c.id IN (
          SELECT conversation_id FROM conversation_participants WHERE user_id = $1
        )
@@ -80,23 +85,27 @@ export async function POST(request: NextRequest) {
       return NextResponse.json({ conversationId: existing.rows[0].id })
     }
 
-    const convResult = await query(
-      `INSERT INTO conversations DEFAULT VALUES RETURNING id`,
-    )
-    const conversationId = convResult.rows[0].id
+    const result = await transaction(async (client) => {
+      const convResult = await client.query(
+        `INSERT INTO conversations DEFAULT VALUES RETURNING id`,
+      )
+      const conversationId = convResult.rows[0].id
 
-    await query(
-      `INSERT INTO conversation_participants (conversation_id, user_id, last_read_at) VALUES ($1, $2, NOW()), ($1, $3, NOW())`,
-      [conversationId, user.id, userId],
-    )
+      await client.query(
+        `INSERT INTO conversation_participants (conversation_id, user_id, last_read_at) VALUES ($1, $2, NOW()), ($1, $3, NOW())`,
+        [conversationId, user.id, userId],
+      )
 
-    const otherUser = await query(
-      `SELECT id, first_name || ' ' || last_name AS name, email, avatar_url
-       FROM users WHERE id = $1`,
-      [userId],
-    )
+      const otherResult = await client.query(
+        `SELECT id, first_name || ' ' || last_name AS name, email, avatar_url
+         FROM users WHERE id = $1`,
+        [userId],
+      )
 
-    const other = otherUser.rows[0]
+      return { conversationId, other: otherResult.rows[0] }
+    })
+
+    const { conversationId, other } = result
 
     return NextResponse.json({
       conversation: {