feat: add real-time cost ticker for Workshop interactions

Adds a live cost ticker (⚡ ~N sats) visible in the top-right HUD
during active paid interactions. The ticker appears when a work
invoice is issued with the estimated cost, updates to the final
charged amount when the job completes, and auto-hides after 5s.

Changes:
- event-bus.ts: add CostEvent type { cost:update, jobId, sats, phase, isFinal }
- events.ts: translate cost:update bus events → cost_update WS messages
- jobs.ts: emit cost:update with estimated sats on invoice creation,
  and again with actual sats when work completes (paid jobs only)
- sessions.ts: emit cost:update with debitedSats after each session request
- ui.js: add showCostTicker/updateCostTicker/hideCostTicker (fixed-position HUD badge)
- websocket.js: handle cost_update messages, drive the cost ticker

Fixes #68

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

artifacts/api-server/src/lib/event-bus.ts

@@ -15,7 +15,10 @@ export type DebateEvent =
   | { type: "debate:argument"; jobId: string; agent: "Beta-A" | "Beta-B"; position: "accept" | "reject"; argument: string }
   | { type: "debate:verdict"; jobId: string; accepted: boolean; reason: string };
 
-export type BusEvent = JobEvent | SessionEvent | DebateEvent;
+export type CostEvent =
+  | { type: "cost:update"; jobId: string; sats: number; phase: "eval" | "work" | "session"; isFinal: boolean };
+
+export type BusEvent = JobEvent | SessionEvent | DebateEvent | CostEvent;
 
 class EventBus extends EventEmitter {
   emit(event: "bus", data: BusEvent): boolean;