feat(ai): v2 新增 GenerateReport RPC + ReportService

第 9 个 RPC GenerateReport（学情报告生成）：data-ana 学情数据 → LLM 生成 → 结构化提取

新增 ReportService 业务编排层 + GenerateReportRequest/GeneratedReport 模型

gRPC servicer + HTTP POST /v1/ai/generate/report（权限 ai:report:generate）

proto_gen 重新生成 + 测试覆盖（servicer/service/HTTP/模型/权限 共 26 用例）

402 测试通过，覆盖率 88.5%

services/ai/tests/test_permission.py

@@ -9,6 +9,7 @@ from src.ai.middleware.permission import (
     PERMISSION_AI_LESSON_CONFIRM,
     PERMISSION_AI_LESSON_GENERATE,
     PERMISSION_AI_QUESTION_GENERATE,
+    PERMISSION_AI_REPORT_GENERATE,
     PermissionGuard,
 )
 
@@ -39,6 +40,7 @@ class TestPermissionGuard:
         guard.check(ctx, PERMISSION_AI_QUESTION_GENERATE)
         guard.check(ctx, PERMISSION_AI_LESSON_GENERATE)
         guard.check(ctx, PERMISSION_AI_LESSON_CONFIRM)
+        guard.check(ctx, PERMISSION_AI_REPORT_GENERATE)
 
     def test_student_only_chat(self) -> None:
         """student 角色仅有 chat 权限."""
@@ -49,6 +51,14 @@ class TestPermissionGuard:
             guard.check(ctx, PERMISSION_AI_QUESTION_GENERATE)
         assert exc_info.value.code == ErrorCode.AI_FORBIDDEN
 
+    def test_student_denied_report(self) -> None:
+        """student 角色无权生成学情报告."""
+        guard = PermissionGuard(dev_mode=False)
+        ctx = UserContext(user_id="u-1", role="student")
+        with pytest.raises(AIError) as exc_info:
+            guard.check(ctx, PERMISSION_AI_REPORT_GENERATE)
+        assert exc_info.value.code == ErrorCode.AI_FORBIDDEN
+
     def test_unknown_role_defaults_student(self) -> None:
         """未知角色降级为 student 权限."""
         guard = PermissionGuard(dev_mode=False)