Add PostgreSQL database layer with Kysely and migrations

- Add database.ts with connection pool, Kysely query builder, and migration runner
- Create migrations for users and sessions tables (0001, 0002)
- Implement PostgresAuthStore to replace InMemoryAuthStore
- Wire up database service in services/index.ts

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

express/migrations/0001_users.sql

@@ -0,0 +1,21 @@
+-- 0001_users.sql
+-- Create users table for authentication
+
+CREATE TABLE users (
+    id UUID PRIMARY KEY,
+    email TEXT UNIQUE NOT NULL,
+    password_hash TEXT NOT NULL,
+    display_name TEXT,
+    status TEXT NOT NULL DEFAULT 'pending',
+    roles TEXT[] NOT NULL DEFAULT '{}',
+    permissions TEXT[] NOT NULL DEFAULT '{}',
+    email_verified BOOLEAN NOT NULL DEFAULT FALSE,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+-- Index for email lookups (login)
+CREATE INDEX users_email_idx ON users (LOWER(email));
+
+-- Index for status filtering
+CREATE INDEX users_status_idx ON users (status);