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

.env.example

@@ -0,0 +1,14 @@
+# Server
+PORT=8080
+GIN_MODE=debug
+SESSION_SECRET=
+
+# Data directory (cloned repos, etc.)
+DATA_DIR=data
+
+# MySQL
+MYSQL_HOST=127.0.0.1
+MYSQL_PORT=3306
+MYSQL_USER=root
+MYSQL_PASSWORD=
+MYSQL_DATABASE=pr_helper