feat(guild-messaging): add gap-detection metadata for seq backfill responses

Fabric.Backend.Guild/src/messaging/messaging.controller.ts

@@ -132,6 +132,26 @@ export class MessagingController {
         ? Math.min(requestedLimit, MAX_PAGE_LIMIT)
         : DEFAULT_PAGE_LIMIT;
 
+    if (from > to) {
+      return {
+        items: [],
+        page: {
+          seqFrom: from,
+          seqTo: to,
+          limit: safeLimit,
+          returned: 0,
+          hasMore: false,
+          nextExpectedSeq: from,
+          highestCommittedSeq: 0,
+        },
+      };
+    }
+
+    const channel = await this.channelRepo.findOne({ where: { id: channelId } });
+    if (!channel) {
+      throw new NotFoundException('channel not found');
+    }
+
     const qb = this.messageRepo
       .createQueryBuilder('m')
       .where('m.channelId = :channelId', { channelId })
@@ -143,6 +163,16 @@ export class MessagingController {
     const rows = await qb.limit(safeLimit).getMany();
     const items = rows.map((m) => this.toView(m));
 
+    let nextExpectedSeq = from;
+    for (const row of rows) {
+      if (row.seq > nextExpectedSeq) {
+        break;
+      }
+      if (row.seq === nextExpectedSeq) {
+        nextExpectedSeq += 1;
+      }
+    }
+
     return {
       items,
       page: {
@@ -151,6 +181,8 @@ export class MessagingController {
         limit: safeLimit,
         returned: items.length,
         hasMore: total > items.length,
+        nextExpectedSeq,
+        highestCommittedSeq: channel.lastSeq,
       },
     };
   }

docs/TODO-backend-center-guild.md

@@ -51,8 +51,8 @@
 - [x] seq 分配改为 DB 原子方案（避免并发冲突）
 
 ### 2.3 一致性与回补
-- [ ] 回补接口：`seq_from/seq_to`
-- [ ] 断片检测辅助响应字段（next_expected_seq 等）
+- [x] 回补接口：`seq_from/seq_to`
+- [x] 断片检测辅助响应字段（next_expected_seq 等）
 - [ ] 幂等键支持（写接口）
 
 ### 2.4 实时通信（MVP 后半）