feat: add ESLint, lint-staged, and update pre-commit hook + CI

- Add ESLint 9 flat config (eslint.config.mjs) with typescript-eslint
  recommended rules and eslint-config-prettier
- Add lint-staged to run eslint+prettier only on staged files
- Update pre-commit hook to use lint-staged instead of full prettier check
- Add `lint` and `format:check` scripts to package.json
- Add Lint step to CI workflow
- Fix resulting lint errors: unused vars (_ctx, _options, catch binding),
  any→unknown in type declarations, stale eslint-disable comments

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/cloudflare-email.ts

@@ -5,7 +5,7 @@ import { processEmail, RawAttachment } from "./email-processor";
 export async function handleCloudflareEmail(
   message: ForwardableEmailMessage,
   env: Env,
-  ctx: ExecutionContext,
+  _ctx: ExecutionContext,
 ): Promise<void> {
   try {
     const email = await PostalMime.parse(message.raw);