feat(sse): implement multiplexed SSE messages

[BastiDood]

This PR implements the unified interface for SSE streams. The new /api/metrics endpoint is now an SSE endpoint that streams UserMessage[] as its first message followed by individual UserMessage objects.

interface Flow {
    ty: 'flow';
    ts: Date;
    flow: number;
}

interface Leak {
    ty: 'leak';
    ts: Date;
}

interface Control {
    ty: 'control';
    ts: Date;
    shutdown: boolean;
}

export type UserMessage = Flow | Leak | Control;