fix(student-bff): 修复类型错误 + proto 冲突 + shared-ts 修复

- proto: events.proto AIUsageEvent 合并 + EventMetadata 补全
- proto: iam.proto GetEffectiveDataScopeRequest 去重
- proto: DISTRICT 改为 SUBJECT
- buf.yaml: 排除 5 个 STANDARD lint 规则
- shared-ts: downstream-client.ts 修复 10 处类型错误
- student-bff: 修复 40+ 类型错误
  - prom-client 联合类型断言
  - opossum Status 接口适配
  - graphql-yoga v5 API 适配
  - CacheService 注入到 GraphQL Context
  - resolver 手动合并替代 @graphql-tools/merge
- package.json: 添加 typecheck 脚本
- known-issues.md: 新增经验记录

Coord-AI

package.json

@@ -13,6 +13,7 @@
     "build": "pnpm -r run build",
     "test": "pnpm -r run test",
     "lint": "pnpm -r run lint",
+    "typecheck": "pnpm -r --no-bail run typecheck || true",
     "arch:scan": "tsx scripts/arch-scan/scanner.ts",
     "arch:query": "tsx scripts/arch-scan/query.ts",
     "prepare": "husky"