feat(telemetry): count appwrite.io WebMCP tool calls - #171

Adds a public /webmcp/events beacon endpoint that accepts bounded, allowlisted WebMCP event data and records calls from https://appwrite.io. Adds WebMCP call and duration telemetry instruments, constants for allowed labels and limits, and end-to-end coverage for event counting, origin filtering, malformed payloads, and preflight requests.