chore: remove cloudflare + docker + legacy migration scripts

Phase 7 cleanup of the Vercel + Upstash consolidation plan:

- delete wrangler.toml, Dockerfile, docker-compose{,.dev}.yml,
  scripts/migrate-atlas-to-upstash.js (one-shot migration done)
- drop wrangler + mongodb devDeps and migrate* npm scripts;
  regenerate package-lock.json (-70 packages)
- prune CF/Wrangler/Atlas-export entries from .gitignore + .vercelignore
- drop MONGODB_URI from .env.deploy.example
- rewrite README for Vercel + Upstash architecture
- refresh stale Cloudflare comments in src/{logger,models,repository}

package.json

@@ -13,14 +13,8 @@
     "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.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",
   "dependencies": {
     "@upstash/redis": "^1.38.0",