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/auth.go

@@ -126,7 +126,7 @@ func (h *AuthHandler) HandleRegister(c *gin.Context) {
 
 	// Seed default settings for the user
 	for key, val := range defaultUserSettings {
-		h.db.Exec(`INSERT OR IGNORE INTO user_settings (user_id, key, value) VALUES (?, ?, ?)`, userID, key, val)
+		h.db.Exec(`INSERT IGNORE INTO user_settings (user_id, key, value) VALUES (?, ?, ?)`, userID, key, val)
 	}
 
 	// Auto-login: set session