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)

internal/config/database.go

@@ -4,15 +4,22 @@ import "fmt"
 
 // DatabaseConfig 数据库配置
 type DatabaseConfig struct {
-	Type                 string         `mapstructure:"type"`
-	SQLite               SQLiteConfig   `mapstructure:"sqlite"`
-	Postgres             PostgresConfig `mapstructure:"postgres"`
-	MaxIdleConns         int            `mapstructure:"max_idle_conns"`
-	MaxOpenConns         int            `mapstructure:"max_open_conns"`
-	LogLevel             string         `mapstructure:"log_level"`
-	SlowThresholdMs      int            `mapstructure:"slow_threshold_ms"`
-	IgnoreRecordNotFound bool           `mapstructure:"ignore_record_not_found"`
-	ParameterizedQueries bool           `mapstructure:"parameterized_queries"`
+	Type                 string                   `mapstructure:"type"`
+	SQLite               SQLiteConfig             `mapstructure:"sqlite"`
+	Postgres             PostgresConfig           `mapstructure:"postgres"`
+	MaxIdleConns         int                      `mapstructure:"max_idle_conns"`
+	MaxOpenConns         int                      `mapstructure:"max_open_conns"`
+	LogLevel             string                   `mapstructure:"log_level"`
+	SlowThresholdMs      int                      `mapstructure:"slow_threshold_ms"`
+	IgnoreRecordNotFound bool                     `mapstructure:"ignore_record_not_found"`
+	ParameterizedQueries bool                     `mapstructure:"parameterized_queries"`
+	// 读写分离：单副本配置（支持环境变量，如 APP_DATABASE_REPLICA_HOST）
+	Replica        PostgresConfig `mapstructure:"replica"`
+	// 读写分离：多副本配置（仅 YAML 支持，环境变量无法覆盖切片结构体）
+	Replicas       []PostgresConfig `mapstructure:"replicas"`
+	ReplicaPolicy  string           `mapstructure:"replica_policy"`
+	ReplicaMaxIdle int              `mapstructure:"replica_max_idle_conns"`
+	ReplicaMaxOpen int              `mapstructure:"replica_max_open_conns"`
 }
 
 // SQLiteConfig SQLite 配置
@@ -20,7 +27,7 @@ type SQLiteConfig struct {
 	Path string `mapstructure:"path"`
 }
 
-// PostgresConfig PostgreSQL 配置
+// PostgresConfig PostgreSQL 配置（主库和副本共用）
 type PostgresConfig struct {
 	Host     string `mapstructure:"host"`
 	Port     int    `mapstructure:"port"`
@@ -37,3 +44,18 @@ func (d PostgresConfig) DSN() string {
 		d.Host, d.Port, d.User, d.Password, d.DBName, d.SSLMode,
 	)
 }
+
+// HasReplica 返回是否配置了读副本（单副本或多副本）
+func (d DatabaseConfig) HasReplica() bool {
+	return d.Replica.Host != "" || len(d.Replicas) > 0
+}
+
+// AllReplicas 返回所有读副本配置（合并单副本 + 多副本）
+func (d DatabaseConfig) AllReplicas() []PostgresConfig {
+	var result []PostgresConfig
+	if d.Replica.Host != "" {
+		result = append(result, d.Replica)
+	}
+	result = append(result, d.Replicas...)
+	return result
+}