feat: let AI chat read lesson video transcripts on demand

Add get_lesson_transcript (teacher) and read_lesson_transcript (student)
tools that fetch a lesson's uploaded-video transcript from media_transcript
by the video assetIds. The agents call them only when a question is about
the video's spoken content, so text-only chats keep a cacheable prefix.

apps/api/src/services/agent/student-tools.ts

@@ -7,6 +7,7 @@ import * as schema from '@cio/db/schema';
 import type { AiTutorSettings } from '@cio/ai-assistant';
 
 import { getLesson } from '@cio/core/services/lesson/lesson';
+import { getLessonVideoTranscript } from '@cio/core/services/agent/lesson-transcript';
 import { getExercise } from '@cio/core/services/exercise/exercise';
 import { listCourseSections } from '@cio/core/services/course/section';
 import { AppError } from '@api/utils/errors';
@@ -191,6 +192,19 @@ export function buildStudentAgentTools(orgId: string, userId: string, courseId:
       }
     }),
 
+    read_lesson_transcript: tool({
+      description:
+        "Read the transcript of a lesson's uploaded video(s). A video's spoken content is NOT part of the lesson body, so use this whenever the learner asks about what the video says, explains, or demonstrates. Only uploaded videos are transcribed — embedded links (YouTube, etc.) return no transcript.",
+      inputSchema: readLessonParam,
+      execute: async (args) => {
+        return executeStudentTool('read_lesson_transcript', { orgId, userId, courseId, args }, async () => {
+          await verifyLessonBelongsToCourse(args.lessonId, courseId);
+
+          return getLessonVideoTranscript(args.lessonId, orgId);
+        });
+      }
+    }),
+
     read_exercise: tool({
       description:
         'Read an exercise prompt — the question text and options visible to a student. Answer keys, correct flags, and marking schemes are stripped before returning.',