fix(platform-service+events): 3 more gaps in diagnostics + delivery

- diagnostics/subscribers: wire session.created email notification to
  target user using existing 'diagnostics-session-created' template
  (was just logging instead of sending the email)
- events/types: add missing 'currency' field to payment.failed schema
  (payment.succeeded had it, payment.failed did not — inconsistency)
- delivery/subscribers: use event.payload.currency instead of hardcoded
  empty string in payment-failed email variables
- Typecheck clean, 1483/1483 tests pass

packages/events/src/types.ts

@@ -120,6 +120,7 @@ export const PlatformEventSchemas = {
     invoiceId: z.string(),
     userId: z.string(),
     amount: z.number(),
+    currency: z.string().default('usd'),
     retryCount: z.number(),
     productId: z.string(),
   }),

services/platform-service/src/modules/delivery/subscribers.ts

@@ -124,7 +124,7 @@ export function registerDeliverySubscribers(
             displayName: user.displayName,
             productName: resolveProductName(productId),
             amount: String(event.payload.amount ?? ''),
-            currency: '',
+            currency: event.payload.currency ?? 'usd',
             billingUrl: `${baseUrl}/billing`,
           },
           productId,

services/platform-service/src/modules/diagnostics/subscribers.ts

@@ -43,12 +43,39 @@ export function registerDiagnosticsSubscribers(
       };
       await auditRepo.create(auditDoc);
 
-      // Send email notification to target user if email is available
-      // Note: In production, we'd look up the user's email from the users repository
-      // For now, we log that notification would be sent
+      // Notify the target user via email if they have an account
+      if (event.payload.targetUserId) {
+        try {
+          const targetUser = await getUserById(event.payload.targetUserId);
+          if (targetUser) {
+            const session = await getSession(event.payload.sessionId);
+            await dispatchEmail(
+              {
+                to: targetUser.email,
+                templateId: 'diagnostics-session-created',
+                variables: {
+                  displayName: targetUser.displayName || targetUser.email.split('@')[0],
+                  productName: event.payload.productId,
+                  sessionId: event.payload.sessionId,
+                  startedAt: new Date().toISOString(),
+                  maxDurationMinutes: String(session?.maxDurationMinutes ?? 30),
+                },
+                productId: event.payload.productId,
+                userId: event.payload.targetUserId,
+              },
+              log
+            );
+          }
+        } catch (notifyErr) {
+          log.error(
+            { notifyErr },
+            '[diagnostics/subscriber] Failed to notify target user on create'
+          );
+        }
+      }
       log.info(
         { sessionId: event.payload.sessionId, targetUserId: event.payload.targetUserId },
-        '[diagnostics/subscriber] Session created, user notification queued'
+        '[diagnostics/subscriber] Session created, user notified'
       );
     } catch (err) {
       log.error(