fix: address PR review comments

- Fix KV json overload to return Promise<unknown | null> (null on missing keys)
- Add shebang to Husky pre-commit hook
- Explicitly add eslint ^10.0.0 to devDependencies

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

package.json

@@ -32,6 +32,7 @@
     "@types/mailparser": "^3.4.6",
     "@types/rss": "^0.0.32",
     "@vitest/coverage-v8": "^4.0.18",
+    "eslint": "^10.0.0",
     "eslint-config-prettier": "^10.1.8",
     "happy-dom": "^20.5.0",
     "husky": "^9.1.7",