feat: migrate database from SQLite to MySQL with .env configuration

- Replace go-sqlite3 with go-sql-driver/mysql (pure Go, no CGO needed)
- Add joho/godotenv for .env file loading
- Rewrite database/db.go with MySQL-compatible DDL (BIGINT AUTO_INCREMENT, InnoDB, utf8mb4)
- Convert all SQLite-specific SQL: INSERT OR IGNORE → INSERT IGNORE,
  INSERT OR REPLACE → INSERT ... ON DUPLICATE KEY UPDATE,
  datetime('now') → NOW(), date arithmetic → DATE_SUB()
- Add MySQL config fields to config.go (host/port/user/password/database)
- Add .env.example with connection parameter template
- Update Dockerfile: remove CGO/gcc/musl dependency, smaller build
- Update docker-compose.yml: add MySQL service container with healthcheck
- Update CLAUDE.md documentation

docker-compose.yml

@@ -1,4 +1,20 @@
 services:
+  mysql:
+    image: mysql:8.0
+    environment:
+      MYSQL_ROOT_PASSWORD: ${MYSQL_PASSWORD:-pr_helper_pass}
+      MYSQL_DATABASE: ${MYSQL_DATABASE:-pr_helper}
+    volumes:
+      - mysql-data:/var/lib/mysql
+    ports:
+      - "3306:3306"
+    healthcheck:
+      test: ["CMD", "mysqladmin", "ping", "-h", "localhost"]
+      interval: 10s
+      timeout: 5s
+      retries: 5
+    restart: unless-stopped
+
   pr-helper:
     build: .
     ports:
@@ -7,7 +23,16 @@ services:
       - pr-helper-data:/app/data
     environment:
       - GIN_MODE=release
+      - MYSQL_HOST=mysql
+      - MYSQL_PORT=3306
+      - MYSQL_USER=root
+      - MYSQL_PASSWORD=${MYSQL_PASSWORD:-pr_helper_pass}
+      - MYSQL_DATABASE=${MYSQL_DATABASE:-pr_helper}
+    depends_on:
+      mysql:
+        condition: service_healthy
     restart: unless-stopped
 
 volumes:
+  mysql-data:
   pr-helper-data: