feat(01-02): create Flyway migrator service and initial migration

- Add Migrator trait with migrate/status ZIO effects
- Create V1 migration for summer_users table
- Configure Flyway with summer_migrations table and classpath location
- Migrations run manually via CLI, not on startup

summercms/resources/db/migration/V1__create_summer_users.sql

@@ -0,0 +1,26 @@
+-- Summer CMS initial schema
+-- Creates the base users table for future auth
+
+CREATE TABLE summer_users (
+    id BIGSERIAL PRIMARY KEY,
+    email VARCHAR(255) NOT NULL UNIQUE,
+    password_hash VARCHAR(255) NOT NULL,
+    created_at TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT NOW(),
+    updated_at TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX idx_summer_users_email ON summer_users(email);
+
+-- Trigger for updated_at
+CREATE OR REPLACE FUNCTION update_updated_at_column()
+RETURNS TRIGGER AS $$
+BEGIN
+    NEW.updated_at = NOW();
+    RETURN NEW;
+END;
+$$ language 'plpgsql';
+
+CREATE TRIGGER update_summer_users_updated_at
+    BEFORE UPDATE ON summer_users
+    FOR EACH ROW
+    EXECUTE FUNCTION update_updated_at_column();