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/routes/sessions.ts

@@ -465,6 +465,11 @@ router.post("/sessions/:id/request", async (req: Request, res: Response) => {
       }
     });
 
+    // Emit real-time cost update for the UI cost ticker (#68)
+    if (finalState === "complete" && debitedSats > 0) {
+      eventBus.publish({ type: "cost:update", jobId: requestId, sats: debitedSats, phase: "session", isFinal: true });
+    }
+
     // ── Trust scoring ────────────────────────────────────────────────────────
     if (session.nostrPubkey) {
       if (finalState === "complete") {