feat: Client-published events + realtime presence

- POST /projects/:id/events accepts client-authored events restricted to
  the custom.* namespace — system types can't be spoofed. They flow
  through the same publishEvent fan-out: realtime, webhooks, and
  event_pattern functions all fire.
- Presence: subscribe/unsubscribe bumps a per-project count and
  broadcasts presence.update {online} to the realtime stream only —
  deliberately not through publishEvent so webhooks and "*" functions
  don't fire on connect noise. GET …/realtime/presence reads the count.
- @primora/client gains publish() and presence(); CLI gains events:send.

Verified live: presence 0->1 on subscribe, presence.update frame
delivered, custom event delivered to SSE + fired a custom.* function,
spoofed document.created rejected 400.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/cli/README.md

@@ -21,7 +21,14 @@ project interactively.
 ## Commands
 
 `orgs`, `projects`, `buckets`, `objects` (list/upload/download/rm), `keys`,
-`jobs`, `audit list --follow`. Every command accepts `--json`.
+`jobs`, `events:send`, `audit list --follow`. Every command accepts `--json`.
+
+```sh
+primora events:send custom.deploy.done --data '{"sha":"abc123"}'
+```
+
+`events:send` publishes a `custom.*` event — realtime subscribers, matching
+webhooks, and `event_pattern` functions all fire.
 
 ```sh
 primora documents list --collection users \