feat: wire up swagger-jsdoc dynamic spec, delete static openapi.json

- Create src/swagger.ts config module for swagger-jsdoc
- Add GET /openapi.json dynamic route (generated from @openapi annotations)
- Delete static public/openapi.json (was drifting from code)
- Add @openapi annotation for deprecated /v1/signup/free in index.ts
- Import swaggerSpec into index.ts
- All 12 endpoints now code-driven: demo/html, demo/markdown, convert/html,
  convert/markdown, convert/url, templates, templates/{id}/render,
  recover, recover/verify, billing/checkout, signup/free, health

package.json

@@ -32,10 +32,11 @@
     "@types/node": "^22.0.0",
     "@types/nodemailer": "^7.0.9",
     "@types/pg": "^8.11.0",
+    "@types/swagger-jsdoc": "^6.0.4",
     "terser": "^5.46.0",
     "tsx": "^4.19.0",
     "typescript": "^5.7.0",
     "vitest": "^3.0.0"
   },
   "type": "module"
-}
+}