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;

artifacts/api-server/src/routes/events.ts

@@ -247,6 +247,16 @@ function translateEvent(ev: BusEvent): object | null {
       };
     }
 
+    // ── Real-time cost ticker (#68) ───────────────────────────────────────────
+    case "cost:update":
+      return {
+        type: "cost_update",
+        jobId: ev.jobId,
+        sats: ev.sats,
+        phase: ev.phase,
+        isFinal: ev.isFinal,
+      };
+
     default:
       return null;
   }

artifacts/api-server/src/routes/jobs.ts

@@ -205,6 +205,8 @@ async function runEvalInBackground(
       // to avoid economic DoS where pool is reserved before the user ever pays.
 
       eventBus.publish({ type: "job:state", jobId, state: "awaiting_work_payment" });
+      // Emit estimated cost so the UI ticker can show ~N sats before payment
+      eventBus.publish({ type: "cost:update", jobId, sats: invoiceSats, phase: "work", isFinal: false });
     } else {
       await db
         .update(jobs)
@@ -307,6 +309,10 @@ async function runWorkInBackground(
       refundState,
     });
     eventBus.publish({ type: "job:completed", jobId, result: workResult.result });
+    // Emit final actual cost for the UI cost ticker
+    if (!isFree && actualAmountSats > 0) {
+      eventBus.publish({ type: "cost:update", jobId, sats: actualAmountSats, phase: "work", isFinal: true });
+    }
 
     // Credit the generosity pool from paid interactions
     if (!isFree && workAmountSats > 0) {

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") {