feat: migrate to Vercel + Upstash with KEY_PREFIX namespacing

Phases 1-5 of consolidate-vercel-upstash plan. Replaces Cloudflare
Workers + KV with Vercel serverless functions + Upstash Redis. Inlines
app-store-scraper / google-play-scraper npm libs (drops the
store-scraper.vercel.app HTTP roundtrip). KEY_PREFIX (default
'store-scraper-bot:') namespaces all Redis keys so the Upstash DB can
be safely shared with other Vercel projects.

- vercel.json + .vercelignore + Vercel-aware package.json scripts
- api/webhook.js + api/cron.js Vercel functions (with shared
  src/app-builder.js); cron auth fails closed when CRON_SECRET unset
- src/repository/upstash.js replaces kv.js; all 4 repos take a handle
  bundling client + prefix
- scripts/migrate-atlas-to-upstash.js writes legacy Java Atlas state
  directly to Upstash with --dry-run + --include-cache flags
- .env.example refreshed for the new env surface

Phases 6 (Vercel deploy + webhook cutover) and 7 (Docker + wrangler
cleanup) remain operator-driven post-deploy.

package.json

@@ -7,19 +7,25 @@
   "engines": {
     "node": ">=20"
   },
-  "main": "src/index.js",
+  "main": "api/webhook.js",
   "scripts": {
-    "dev": "wrangler dev",
-    "deploy": "wrangler deploy && npm run register",
+    "dev": "vercel dev",
+    "deploy": "vercel deploy --prod && npm run register",
     "register": "node --env-file=.env.deploy scripts/register-webhook.js",
     "register:dry": "node --env-file=.env.deploy scripts/register-webhook.js --dry-run",
-    "migrate": "node --env-file=.env scripts/migrate-atlas-to-kv.js",
-    "migrate:bulk": "wrangler kv bulk put --binding STORE_KV --remote scripts/.atlas-export.json",
+    "migrate": "node --env-file=.env.deploy scripts/migrate-atlas-to-upstash.js",
+    "migrate:dry": "node --env-file=.env.deploy scripts/migrate-atlas-to-upstash.js --dry-run",
     "lint": "node scripts/check-secret-leaks.js"
   },
   "devDependencies": {
     "mongodb": "^6.10.0",
     "wrangler": "^3.90.0"
   },
-  "license": "Apache-2.0"
+  "license": "Apache-2.0",
+  "dependencies": {
+    "@upstash/redis": "^1.38.0",
+    "@vercel/functions": "^3.5.1",
+    "app-store-scraper": "^0.18.0",
+    "google-play-scraper": "^10.1.2"
+  }
 }