feat(portal): 고객 포털 인증과 소셜 로그인 기반 추가

db/migrations/V016__CreatePortalUsers.sql

@@ -0,0 +1,14 @@
+CREATE TABLE IF NOT EXISTS portal_users (
+    id SERIAL PRIMARY KEY,
+    client_id INT NULL REFERENCES clients(id) ON DELETE SET NULL,
+    email VARCHAR(255) NOT NULL UNIQUE,
+    name VARCHAR(100) NOT NULL,
+    phone VARCHAR(50),
+    provider VARCHAR(30) NOT NULL DEFAULT 'local',
+    provider_id VARCHAR(200),
+    password_hash TEXT,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX IF NOT EXISTS idx_portal_users_provider
+    ON portal_users(provider, provider_id);