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

handlers/settings.go

@@ -56,7 +56,7 @@ func (h *SettingsHandler) UpdateSettings(c *gin.Context) {
 		return
 	}
 	for key, val := range body {
-		_, err := tx.Exec(`INSERT OR REPLACE INTO user_settings (user_id, key, value) VALUES (?, ?, ?)`, user.ID, key, val)
+		_, err := tx.Exec(`INSERT INTO user_settings (user_id, key, value) VALUES (?, ?, ?) ON DUPLICATE KEY UPDATE value = VALUES(value)`, user.ID, key, val)
 		if err != nil {
 			tx.Rollback()
 			c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})