feat(api): add versioned REST API with OpenAPI 3.1 spec

Expose /api/v1/* for feed and email management (feeds CRUD, email
list/get/delete, stats) so the service can be automated without scraping
the admin UI. Built on @hono/zod-openapi; the OpenAPI 3.1 spec is served at
/api/openapi.json with a Scalar reference at /api/docs.

Auth is token-based (Authorization: Bearer <ADMIN_PASSWORD>) plus the
existing reverse-proxy headers — no cookie, no CSRF. Extracted the auth
primitives into src/lib/auth.ts and the feed create/update/delete
orchestration into src/lib/feed-service.ts so the admin UI and the REST API
share a single source of truth.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

package.json

@@ -51,7 +51,9 @@
     "wrangler": "4.94.0"
   },
   "dependencies": {
+    "@hono/zod-openapi": "^1.4.0",
     "@hono/zod-validator": "^0.8.0",
+    "@scalar/hono-api-reference": "^0.10.19",
     "escape-html": "^1.0.3",
     "feed": "5.2.1",
     "hono": "4.12.22",