feat: add redis cache and fanout support

docs/sql/IM-SeqId-BIGINT与Redis缓存改造SQL.sql

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+-- IM SeqId BIGINT upgrade for existing MySQL tenant databases.
+-- New schemas have been updated in server/db/mysql/adapter.go and group_schema.go.
+
+ALTER TABLE im_topics MODIFY seqid BIGINT NOT NULL DEFAULT 0;
+ALTER TABLE im_subscriptions MODIFY recvseqid BIGINT DEFAULT 0;
+ALTER TABLE im_subscriptions MODIFY readseqid BIGINT DEFAULT 0;
+ALTER TABLE im_messages MODIFY seqid BIGINT NOT NULL;
+ALTER TABLE im_message_revisions MODIFY seqid BIGINT NOT NULL;
+ALTER TABLE im_favorites MODIFY source_seq BIGINT NOT NULL;
+ALTER TABLE im_abuse_reports MODIFY target_seq BIGINT NOT NULL DEFAULT 0;
+ALTER TABLE im_call_records MODIFY seq_id BIGINT NOT NULL;
+ALTER TABLE im_group_files MODIFY source_seq BIGINT NOT NULL DEFAULT 0;

docs/sql/IM-群消息拆表SQL.sql

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+CREATE TABLE IF NOT EXISTS im_group_messages (
+  id BIGINT NOT NULL AUTO_INCREMENT,
+  topic VARCHAR(64) NOT NULL COMMENT '群 Topic，如 grp_xxx',
+  group_id BIGINT NOT NULL COMMENT '业务群 ID，便于分区、治理和统计',
+  seqid BIGINT NOT NULL COMMENT '群内消息序号，由 Redis 原子发号',
+  from_user BIGINT NOT NULL COMMENT '发送人用户 ID',
+  client_msg_id VARCHAR(64) NULL COMMENT '客户端幂等消息 ID',
+  msg_type VARCHAR(32) NOT NULL DEFAULT 'text' COMMENT 'text/image/file/audio/video/system 等',
+  status VARCHAR(32) NOT NULL DEFAULT 'normal' COMMENT 'normal/recalled/deleted/blocked',
+  content JSON NULL COMMENT '消息正文或 Drafty 内容',
+  head JSON NULL COMMENT '消息头，包含 @、引用、客户端能力等',
+  mentioned_user_ids JSON NULL COMMENT '@ 用户列表，便于提醒和检索',
+  mention_all TINYINT(1) NOT NULL DEFAULT 0 COMMENT '是否 @ 全体',
+  reply_seqid BIGINT NULL COMMENT '回复的群消息 seq',
+  root_seqid BIGINT NULL COMMENT '线程或合并上下文根消息 seq',
+  file_count INT NOT NULL DEFAULT 0 COMMENT '附件数量',
+  content_size INT NOT NULL DEFAULT 0 COMMENT '正文 JSON 序列化后的字节数',
+  has_risk TINYINT(1) NOT NULL DEFAULT 0 COMMENT '是否命中风控或内容安全',
+  audit_status VARCHAR(32) NOT NULL DEFAULT 'pass' COMMENT 'pass/pending/reject',
+  version INT NOT NULL DEFAULT 1 COMMENT '编辑版本',
+  recalled_at DATETIME(3) NULL,
+  recalled_by BIGINT NULL,
+  deleted_at DATETIME(3) NULL,
+  expires_at DATETIME(3) NULL COMMENT '可选保留期，便于过期归档或清理',
+  created_at DATETIME(3) NOT NULL,
+  updated_at DATETIME(3) NOT NULL,
+  PRIMARY KEY (id),
+  UNIQUE KEY uk_im_group_messages_group_seq (group_id, seqid),
+  UNIQUE KEY uk_im_group_messages_topic_seq (topic, seqid),
+  UNIQUE KEY uk_im_group_messages_topic_sender_client_mid (topic, from_user, client_msg_id),
+  KEY idx_im_group_messages_group_created (group_id, created_at),
+  KEY idx_im_group_messages_group_status_seq (group_id, status, seqid),
+  KEY idx_im_group_messages_topic_created (topic, created_at),
+  KEY idx_im_group_messages_topic_from_seq (topic, from_user, seqid),
+  KEY idx_im_group_messages_audit (audit_status, created_at),
+  KEY idx_im_group_messages_expires (expires_at)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;