feat(db): implement read-write splitting for postgres

Add support for database read replicas using the GORM dbresolver plugin. This allows for scaling read operations by distributing them across one or multiple replica nodes.

- Update `DatabaseConfig` to support single and multiple replica configurations.
- Add configuration options for replica connection pooling and selection policy.
- Integrate `dbresolver` in the database initialization process for PostgreSQL.
- Add helper methods to aggregate replica configurations.

internal/config/config.go

@@ -59,6 +59,7 @@ func Load(configPath string) (*Config, error) {
 	viper.SetDefault("database.slow_threshold_ms", 200)
 	viper.SetDefault("database.ignore_record_not_found", true)
 	viper.SetDefault("database.parameterized_queries", true)
+	viper.SetDefault("database.replica_policy", "random")
 	viper.SetDefault("redis.type", "miniredis")
 	viper.SetDefault("redis.redis.host", "localhost")
 	viper.SetDefault("redis.redis.port", 6379)