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.

configs/config.yaml

@@ -21,6 +21,28 @@ database:
     password: postgres
     dbname: with_you
     sslmode: disable
+  # 读写分离配置（仅 postgres 模式生效，不配置则所有请求走主库）
+  # 单副本配置（支持环境变量）:
+  #   APP_DATABASE_REPLICA_HOST, APP_DATABASE_REPLICA_PORT, APP_DATABASE_REPLICA_USER,
+  #   APP_DATABASE_REPLICA_PASSWORD, APP_DATABASE_REPLICA_DBNAME, APP_DATABASE_REPLICA_SSLMODE
+  # replica:
+  #   host: replica1.internal
+  #   port: 5432
+  #   user: readonly
+  #   password: readonly_pass
+  #   dbname: with_you
+  #   sslmode: disable
+  # 多副本配置（仅 YAML，环境变量不支持切片结构体）:
+  # replicas:
+  #   - host: replica1.internal
+  #     port: 5432
+  #     user: readonly
+  #     password: readonly_pass
+  #     dbname: with_you
+  #     sslmode: disable
+  # replica_policy: random  # random
+  # replica_max_idle_conns: 10
+  # replica_max_open_conns: 100
   max_idle_conns: 10
   max_open_conns: 100
   log_level: warn