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/0002_sessions.sql

@@ -0,0 +1,24 @@
+-- 0002_sessions.sql
+-- Create sessions table for auth tokens
+
+CREATE TABLE sessions (
+    token_id TEXT PRIMARY KEY,
+    user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    token_type TEXT NOT NULL,
+    auth_method TEXT NOT NULL,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    expires_at TIMESTAMPTZ NOT NULL,
+    last_used_at TIMESTAMPTZ,
+    user_agent TEXT,
+    ip_address TEXT,
+    is_used BOOLEAN DEFAULT FALSE
+);
+
+-- Index for user session lookups (logout all, etc.)
+CREATE INDEX sessions_user_id_idx ON sessions (user_id);
+
+-- Index for expiration cleanup
+CREATE INDEX sessions_expires_at_idx ON sessions (expires_at);
+
+-- Index for token type filtering
+CREATE INDEX sessions_token_type_idx ON sessions (token_type);