feat: complete Phase 2 tech debt remediation

- Extract shared RSS/Atom fetch logic into feed-fetcher utility (P1-3)
- Split email-processor into validateEmail/storeEmail functions (P1-6)
- Add stateless HMAC-SHA256 CSRF protection to admin forms (P2-8)
- Fix Hono<{ Bindings: Env }> type safety across all routes (P3-13)
- Add entries.test.ts and files.test.ts with full coverage (P1-7)

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

src/types/hono.d.ts

@@ -4,5 +4,6 @@ import { Env } from "./index";
 declare module "hono" {
   interface ContextVariableMap {
     env: Env;
+    csrfToken: string;
   }
 }