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

config/config.go

@@ -3,8 +3,11 @@ package config
 import (
 	"crypto/rand"
 	"encoding/hex"
+	"fmt"
 	"os"
 	"path/filepath"
+
+	"github.com/joho/godotenv"
 )
 
 type Config struct {
@@ -12,14 +15,27 @@ type Config struct {
 	GinMode       string
 	DataDir       string
 	SessionSecret string
+	MySQLHost     string
+	MySQLPort     string
+	MySQLUser     string
+	MySQLPassword string
+	MySQLDatabase string
 }
 
 func Load() *Config {
+	// Load .env file if present (ignore error if missing)
+	godotenv.Load()
+
 	cfg := &Config{
 		Port:          getEnv("PORT", "8080"),
 		GinMode:       getEnv("GIN_MODE", "debug"),
 		DataDir:       getEnv("DATA_DIR", "data"),
 		SessionSecret: getEnv("SESSION_SECRET", ""),
+		MySQLHost:     getEnv("MYSQL_HOST", "127.0.0.1"),
+		MySQLPort:     getEnv("MYSQL_PORT", "3306"),
+		MySQLUser:     getEnv("MYSQL_USER", "root"),
+		MySQLPassword: getEnv("MYSQL_PASSWORD", ""),
+		MySQLDatabase: getEnv("MYSQL_DATABASE", "pr_helper"),
 	}
 	// Generate random session secret if not provided
 	if cfg.SessionSecret == "" {
@@ -33,8 +49,10 @@ func Load() *Config {
 	return cfg
 }
 
-func (c *Config) DBPath() string {
-	return filepath.Join(c.DataDir, "pr-helper.db")
+// MySQLDSN returns the MySQL Data Source Name for go-sql-driver/mysql.
+func (c *Config) MySQLDSN() string {
+	return fmt.Sprintf("%s:%s@tcp(%s:%s)/%s?charset=utf8mb4&parseTime=True&loc=Local",
+		c.MySQLUser, c.MySQLPassword, c.MySQLHost, c.MySQLPort, c.MySQLDatabase)
 }
 
 func (c *Config) ReposDir() string {