Bug: Missing Template for `PLATFORM_REGISTRATION_CANCELLED` Emails

### Description

The email service is failing to send notifications when a platform registration is cancelled. The system attempts to process the incoming Pub/Sub message but encounters a `FileNotFoundError` because the required Markdown template (`strr-PLATFORM_REGISTRATION_CANCELLED.md`) is missing from the `email-templates` directory. This is causing a **500 Internal Server Error** on the `strr-email` service.

> Edit: found another missing template: `strr-STRATA_HOTEL_REGISTRATION_ACTIVE.md`

---

### Error Logs

GCP Logs Explorer link

```json
2026-06-04 13:49:12.385 localhost strr_email[10] ERROR Exception on / [POST]
Traceback (most recent call last):
...
return io.open(self, mode, buffering, encoding, errors, newline)
FileNotFoundError: [Errno 2] No such file or directory: 'email-templates/strr-PLATFORM_REGISTRATION_CANCELLED.md'
```

---

### Steps to Reproduce

1. Trigger a platform registration cancellation event from the `strr-api` service.
2. The `strr-emailer--sub` subscription delivers the payload to the email service.
3. Observe the `strr-email-` service logs.

### Expected Result

The email service successfully locates the `strr-PLATFORM_REGISTRATION_CANCELLED.md` template, injects the context variable (`customContent`, `registrationNumber`), and dispatches the email.

### Actual Result

The application crashes with a `FileNotFoundError` and returns an HTTP 500 status code back to the message broker.

---
### Proposed Solution

1. Create the missing template files:
1. `email-templates/strr-PLATFORM_REGISTRATION_CANCELLED.md`.
2. `email-templates/strr-STRATA_HOTEL_REGISTRATION_ACTIVE.md`

Start in the email-templates directory and inspect the email service flow that handles the incoming Pub/Sub payload. Create strr-PLATFORM_REGISTRATION_CANCELLED.md and strr-STRATA_HOTEL_REGISTRATION_ACTIVE.md using the expected customContent and registrationNumber context, then verify both events no longer produce FileNotFoundError or an HTTP 500 response.

Written by the indexing model from the issue text.