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-playback/src/main/java/com/fruitshorts/playback/dal/entity/DownloadLicense.java

@@ -15,7 +15,7 @@ import java.time.LocalDateTime;
 @TableName("download_license")
 public class DownloadLicense {
 
-    @TableId(type = IdType.AUTO)
+    @TableId(type = IdType.ASSIGN_ID)
     private Long id;
     private Long userId;
     private Long dramaId;

fruitshorts-module-playback/src/main/java/com/fruitshorts/playback/dal/entity/DownloadTask.java

@@ -18,7 +18,7 @@ import java.time.LocalDateTime;
 @TableName("download_task")
 public class DownloadTask {
 
-    @TableId(type = IdType.AUTO)
+    @TableId(type = IdType.ASSIGN_ID)
     private Long id;
     private Long userId;
     private String deviceId;

fruitshorts-module-playback/src/main/java/com/fruitshorts/playback/dal/entity/PlaybackTicket.java

@@ -15,7 +15,7 @@ import java.time.LocalDateTime;
 @TableName("playback_ticket")
 public class PlaybackTicket {
 
-    @TableId(type = IdType.AUTO)
+    @TableId(type = IdType.ASSIGN_ID)
     private Long id;
     private String ticketNo;
     private Long userId;

fruitshorts-module-playback/src/main/java/com/fruitshorts/playback/dal/entity/UserFavorite.java

@@ -15,7 +15,7 @@ import java.time.LocalDateTime;
 @TableName("user_favorite")
 public class UserFavorite {
 
-    @TableId(type = IdType.AUTO)
+    @TableId(type = IdType.ASSIGN_ID)
     private Long id;
     private Long userId;
     private Long dramaId;

fruitshorts-module-playback/src/main/java/com/fruitshorts/playback/dal/entity/WatchHistory.java

@@ -16,7 +16,7 @@ import java.time.LocalDateTime;
 @TableName("watch_history")
 public class WatchHistory {
 
-    @TableId(type = IdType.AUTO)
+    @TableId(type = IdType.ASSIGN_ID)
     private Long id;
     private Long userId;
     private Long dramaId;

fruitshorts-module-playback/src/main/java/com/fruitshorts/playback/dal/entity/WatchProgress.java

@@ -15,7 +15,7 @@ import java.time.LocalDateTime;
 @TableName("watch_progress")
 public class WatchProgress {
 
-    @TableId(type = IdType.AUTO)
+    @TableId(type = IdType.ASSIGN_ID)
     private Long id;
     private Long userId;
     private Long dramaId;