feat: S0 主键改雪花（DDL 去 identity + 实体 ASSIGN_ID + 雪花配置）

为什么：自增主键把 ID 分配锁在单库序列上，分库分表时无法横向扩展。趁线上业务数据
近零（只有系统种子表有量）先改，代价最低。

- sql/schema.sql、sql/schema-phase1.sql、sql/partition-maintenance.sql：61 处
  GENERATED BY DEFAULT AS IDENTITY 去掉，主键改 BIGINT 无默认值
- 52 个实体 @TableId IdType.AUTO -> IdType.ASSIGN_ID；两个入口 id-type: assign_id，
  并加 global-config.sequence.worker-id/datacenter-id（单机固定值，多实例需按实例分配）
- 新增 sql/migrate-s0-snowflake.sql：存量库幂等迁移，走 DROP IDENTITY 保留数据，不重建表
- 实测：三份建表脚本在新库连跑两遍幂等；线上 57 张表 identity 归零；
  watch_history/playback_ticket/order_event 的主键与唯一索引仍含分区键

fruitshorts-module-content/src/main/java/com/fruitshorts/content/dal/entity/ContentBanner.java

@@ -14,7 +14,7 @@ import java.time.LocalDateTime;
 @TableName("content_banner")
 public class ContentBanner {
 
-    @TableId(type = IdType.AUTO)
+    @TableId(type = IdType.ASSIGN_ID)
     private Long id;
     private String imageUrl;
     /** 展示位置，列名为 slot（position 在 PostgreSQL 里是关键字） */

fruitshorts-module-content/src/main/java/com/fruitshorts/content/dal/entity/Drama.java

@@ -14,7 +14,7 @@ import java.time.LocalDateTime;
 @TableName("drama")
 public class Drama {
 
-    @TableId(type = IdType.AUTO)
+    @TableId(type = IdType.ASSIGN_ID)
     private Long id;
     private String code;
     private String title;

fruitshorts-module-content/src/main/java/com/fruitshorts/content/dal/entity/DramaCategory.java

@@ -9,7 +9,7 @@ import lombok.Data;
 @TableName("drama_category")
 public class DramaCategory {
 
-    @TableId(type = IdType.AUTO)
+    @TableId(type = IdType.ASSIGN_ID)
     private Long id;
     private String name;
     private Integer sort;

fruitshorts-module-content/src/main/java/com/fruitshorts/content/dal/entity/DramaEpisode.java

@@ -14,7 +14,7 @@ import java.time.LocalDateTime;
 @TableName("drama_episode")
 public class DramaEpisode {
 
-    @TableId(type = IdType.AUTO)
+    @TableId(type = IdType.ASSIGN_ID)
     private Long id;
     private Long dramaId;
     private Integer epNo;

fruitshorts-module-content/src/main/java/com/fruitshorts/content/dal/entity/DramaReviewRecord.java

@@ -11,7 +11,7 @@ import java.time.LocalDateTime;
 @TableName("drama_review_record")
 public class DramaReviewRecord {
 
-    @TableId(type = IdType.AUTO)
+    @TableId(type = IdType.ASSIGN_ID)
     private Long id;
     private String bizType;
     private Long bizId;