feat(GRO-106): inbound Telnyx webhook + persistence

- Add POST /api/webhooks/telnyx/messaging route with HMAC signature verification
- Add services/messaging/inbound.ts: findOrCreateConversation, upsertMessage (idempotent on providerMessageId), delivery receipt handling
- Register telnyxWebhooksRouter in index.ts (before auth middleware)
- Add unit tests for signature validation, find-or-create, idempotent insert, delivery receipt

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/api/src/index.ts

@@ -29,6 +29,7 @@ import { devRouter } from "./routes/dev.js";
 import { adminSeedRouter } from "./routes/admin/seed.js";
 import { startReminderScheduler } from "./services/reminders.js";
 import { webhooksRouter } from "./routes/stripe-webhooks.js";
+import { telnyxWebhooksRouter } from "./routes/webhooks/telnyx.js";
 
 const app = new Hono();
 
@@ -69,6 +70,9 @@ app.route("/api/portal", portalRouter);
 // Public Stripe webhook endpoint — signature-verified, no auth required
 app.route("/api/webhooks/stripe", webhooksRouter);
 
+// Public Telnyx messaging webhook — signature-verified, no auth required
+app.route("/api/webhooks/telnyx", telnyxWebhooksRouter);
+
 // Dev/demo routes — config is always public, users endpoint is guarded internally
 app.route("/api/dev", devRouter);