feat(observability): add Langfuse tracing + fix templates API

- Add Langfuse OpenTelemetry integration for AI observability
  - All 6 streamText/generateText calls now have experimental_telemetry
  - instrumentation.ts registers LangfuseSpanProcessor alongside Sentry
  - Sentry configured with skipOpenTelemetrySetup to avoid conflicts
  - Requires LANGFUSE_SECRET_KEY, LANGFUSE_PUBLIC_KEY, LANGFUSE_BASEURL envs

- Fix templates API returning empty for system templates
  - GET filtered by tenant_id which system templates don't have
  - Changed to: tenant_id = current OR tenant_id IS NULL
  - POST now includes tenant_id for user-created templates

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: guillermoscript

app/actions/exam-grading.ts

@@ -367,6 +367,7 @@ Evaluate these free-text answers now:`
       prompt: aiPrompt,
       temperature: 0.3, // Lower temperature for more consistent grading
       topP: 0.8,
+      experimental_telemetry: { isEnabled: true, functionId: 'exam-grading', metadata: { examId: String(params.examId), submissionId: String(params.submissionId) } },
     })
 
     const text = result.text

app/api/chat/aristotle/route.ts

@@ -234,6 +234,7 @@ export async function POST(req: Request) {
         model: AI_MODELS.aristotle,
         system: systemPrompt,
         messages: await convertToModelMessages(messages),
+        experimental_telemetry: { isEnabled: true, functionId: 'aristotle-assistant', metadata: { userId: user.id, tenantId, contextPage: contextPage || '' } },
         onFinish: async (event) => {
             if (event.text) {
                 await supabase.from('aristotle_messages').insert({

app/api/chat/exercises/student/route.ts

@@ -44,6 +44,7 @@ export async function POST(req: Request) {
         system: PROMPTS.exerciseCoach(exercise),
         messages: await convertToModelMessages(messages),
         tools: createAITools(supabase, { exerciseId, userId: user.id, tenantId, exerciseType: (exercise as any).exercise_type }),
+        experimental_telemetry: { isEnabled: true, functionId: 'exercise-coach', metadata: { exerciseId: String(exerciseId), userId: user.id, tenantId } },
         onFinish: async (event) => {
             await supabase.from('exercise_messages').insert({
                 exercise_id: exerciseId,

app/api/chat/lesson-task/route.ts

@@ -60,6 +60,7 @@ export async function POST(req: Request) {
         system: PROMPTS.lessonTutor(lesson, aiTask),
         messages: await convertToModelMessages(messages),
         tools: createAITools(supabase, { lessonId, userId: user.id, courseId: lesson.course_id, tenantId }),
+        experimental_telemetry: { isEnabled: true, functionId: 'lesson-tutor', metadata: { lessonId: String(lessonId), userId: user.id, tenantId } },
         onFinish: async (event) => {
             const messageData: any = {
                 lesson_id: lessonId,

app/api/teacher/preview/exercise/route.ts

@@ -18,6 +18,7 @@ export async function POST(req: Request) {
     model: AI_MODELS.coach,
     system: PROMPTS.previewExercise(instructions, system_prompt),
     messages: await convertToModelMessages(messages),
+    experimental_telemetry: { isEnabled: true, functionId: 'preview-exercise', metadata: { userId: user.id } },
     stopWhen: stepCountIs(AI_CONFIG.maxSteps),
   })
 

app/api/teacher/preview/lesson-task/route.ts

@@ -18,6 +18,7 @@ export async function POST(req: Request) {
     model: AI_MODELS.tutor,
     system: PROMPTS.previewLesson(task_description, system_prompt),
     messages: await convertToModelMessages(messages),
+    experimental_telemetry: { isEnabled: true, functionId: 'preview-lesson-task', metadata: { userId: user.id } },
   })
 
   return result.toUIMessageStreamResponse()

app/api/teacher/templates/route.ts

@@ -14,10 +14,11 @@ export async function GET(req: NextRequest) {
   const category = searchParams.get('category')
   const search = searchParams.get('search')
 
+  // System templates have no tenant_id; user templates are scoped to tenant
   let query = supabase
     .from('prompt_templates')
     .select('*')
-    .eq('tenant_id', tenantId)
+    .or(`tenant_id.eq.${tenantId},tenant_id.is.null`)
     .or(`created_by.eq.${user.id},is_system.eq.true`)
     .order('is_system', { ascending: false })
     .order('created_at', { ascending: false })
@@ -34,6 +35,7 @@ export async function GET(req: NextRequest) {
 // POST /api/teacher/templates
 export async function POST(req: NextRequest) {
   const supabase = await createClient()
+  const tenantId = await getCurrentTenantId()
   const { data: { user } } = await supabase.auth.getUser()
 
   if (!user) return new Response('Unauthorized', { status: 401 })
@@ -45,7 +47,8 @@ export async function POST(req: NextRequest) {
     .insert({
       ...body,
       created_by: user.id,
-      is_system: false
+      is_system: false,
+      tenant_id: tenantId,
     })
     .select()
     .single()

instrumentation.ts

@@ -3,6 +3,23 @@ import * as Sentry from "@sentry/nextjs";
 export async function register() {
   if (process.env.NEXT_RUNTIME === "nodejs") {
     await import("./sentry.server.config");
+
+    // Langfuse AI observability via OpenTelemetry
+    const { LangfuseSpanProcessor } = await import("@langfuse/otel");
+    const { NodeTracerProvider } = await import("@opentelemetry/sdk-trace-node");
+
+    const langfuseSpanProcessor = new LangfuseSpanProcessor({
+      // Only export AI SDK spans, not Next.js infra spans
+      shouldExportSpan: (span) => {
+        return span.otelSpan.instrumentationScope.name !== "next.js";
+      },
+    });
+
+    const tracerProvider = new NodeTracerProvider({
+      spanProcessors: [langfuseSpanProcessor],
+    });
+
+    tracerProvider.register();
   }
 
   if (process.env.NEXT_RUNTIME === "edge") {