perf(backfill): stop the prefetch worker from throttling itself (Tier 1)

The worker's own upstream fetches called touchActivity(), setting the
prefetch:activity:<source> cooldown key (TTL 300s) that checkCooldown then
honoured — so after each fetch the worker paused itself for up to ~5 minutes
(re-checking every 60s, ~5 empty cycles per key). At ~1 fetch / 5 min the
3387-job backlog needed ~6 days to drain.

- Wrap each worker job in an AsyncLocalStorage backfill context; touchActivity
  skips the cooldown key when invoked from the worker, so the cooldown reflects
  only real user requests (worker yields to users, never to itself).
- Cooldown TTL 300s -> 90s (a request 5 min ago isn't "active").
- checkCooldown pauses for the key's actual remaining TTL (one wait) instead of
  a fixed 60s re-check loop.

No extra upstream load — only removes the worker's self-imposed idle time.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/api/src/integrations/emex/emex.service.ts

@@ -20,6 +20,7 @@ import {
 import { ConfigService } from "@nestjs/config";
 
 import { ProxyAgent } from "undici";
+import { isBackfillContext } from "../../jobs/prefetch-context";
 import { RedisService } from "../../redis/redis.service";
 import { EmexBrowserService } from "./emex.browser";
 import { createEmptyDecodedVehicle, mapEmexResponse } from "./emex.mapper";
@@ -143,8 +144,11 @@ export class EmexService {
 
   /** Mark EMEX as actively used (5min TTL) to defer prefetch worker */
   private async touchActivity(): Promise<void> {
+    // Worker-originated fetches must NOT register as user activity, or the
+    // backfill worker throttles itself via checkCooldown.
+    if (isBackfillContext()) return;
     try {
-      await this.redis.set("prefetch:activity:emex", String(Date.now()), 300);
+      await this.redis.set("prefetch:activity:emex", String(Date.now()), 90);
     } catch {
       // Non-critical — don't break the request
     }