Pub/Sub for SSE channels

[bpamiri]

Phase 3 — Modernization Target #10

Add a channel abstraction over the existing SSE implementation with pub/sub support.

Design

// Controller — subscribe
function notifications() {
    subscribeToChannel(channel="user.#params.userId#", events="notification,alert");
}

// Publish from anywhere
publish(channel="user.42", event="notification", data=SerializeJSON({
    title: "New message", body: "You have a new message"
}));

• In-memory pub/sub for single-server (wheels.Channel)
• DatabaseAdapter for multi-server via _wheels_events table
• subscribeToChannel() controller method wrapping initSSEStream()
• Global publish() function
• Ship wheels-sse.js client library for auto-reconnect

Backwards Compatibility

Existing renderSSE() / initSSEStream() / sendSSEEvent() API continues to work. Channels are a higher-level abstraction.

See design_docs/MODERNIZE-WHEELS-RIM.md §4.8 for full implementation plan.