[REQUEST] Add a Realtime module to the pure-Dart `insforge` package (parity with `@insforge/sdk` and the Kotlin SDK)

[warcayac]

We build Flutter apps against InsForge tenant backends using the pure-Dart insforge
package (deliberately chosen over insforge_flutter so we can control session storage
ourselves via the injectable SessionStorage interface). The backend's realtime
feature is already provisioned server-side for our tenants (channel patterns such as
authz_user:% with realtime.publish triggers, RLS-governed channel access).

Gap

• The TypeScript SDK (@insforge/sdk) and the Kotlin SDK both expose a realtime
  module: connect, subscribe, publish, on, disconnect, plus presence.
• The pure-Dart package (0.1.0) has no realtime module at all.

Use case (concrete)

Postgres-changes-style authorization watch: after sign-in, a Flutter app must react
immediately when an admin changes the user's role/status (e.g., suspension) so the
session can be force-refreshed. The backend publishes those events on the per-user
channel authz_user:{userId}. Without an SDK realtime module, clients are forced to
poll the public.t_get_authz_snapshot RPC on an interval — adding latency (up to one
poll interval before a suspended user is locked out) and consuming egress budget,
which the SDK docs themselves discourage ("never poll unbounded on an interval").

Request

A realtime module in the pure-Dart package mirroring the JS/Kotlin surface:

1. connect/disconnect lifecycle with channel subscribe/unsubscribe.
2. Event callbacks (on) for channel messages.
3. Presence snapshots and join/leave deltas, if the backend contract matches the JS SDK.
4. Transparent reuse of the authenticated session (access token / refresh via the
   existing AuthClient + SessionStorage wiring), so realtime works both for
   anonymous and authenticated channels.
5. Keep it consistent with the existing package style (dio-based HTTP client,
   injectable seams), so it remains usable without insforge_flutter.

Alternatives (non-preferred)

• Raw WebSocket integration per app duplicates SDK responsibilities (auth headers,
  reconnect/backoff, channel protocol) and drifts from the platform contract as it
  evolves.