feat: replace MongoDB driver with Cloudflare KV storage

Drops the mongodb dependency entirely; all four logical collections
(admin singleton, group, apple_app, google_app) now live in a single KV
namespace bound as STORE_KV with prefixed keys. Cache TTL is delegated
to KV via expirationTtl (clamped to the 60s minimum). Document shape,
field names, and Java parity at the doc level are preserved.

- Adds src/repository/kv.js helper (getJson/putJson/del with TTL clamp)
- Rewrites all four *-repository.js modules on top of KV
- Removes src/repository/mongodb.js and the MONGODB_URI env requirement
- Adds an early STORE_KV-binding guard in src/index.js
- Bumps to 0.3.0

package.json

@@ -1,7 +1,7 @@
 {
   "name": "js-store-scraper-bot",
-  "version": "0.2.0",
-  "description": "JavaScript port of store-scraper-bot — Telegram bot tracking Apple App Store + Google Play app updates. Deploys to Cloudflare Workers.",
+  "version": "0.3.0",
+  "description": "JavaScript port of store-scraper-bot — Telegram bot tracking Apple App Store + Google Play app updates. Deploys to Cloudflare Workers, backed by Cloudflare KV.",
   "type": "module",
   "private": true,
   "engines": {
@@ -15,9 +15,6 @@
     "register:dry": "node --env-file=.env.deploy scripts/register-webhook.js --dry-run",
     "lint": "node scripts/check-secret-leaks.js"
   },
-  "dependencies": {
-    "mongodb": "^6.10.0"
-  },
   "devDependencies": {
     "wrangler": "^3.90.0"
   },