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-lock.json

@@ -28,6 +28,7 @@
         "@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",
@@ -1170,6 +1171,13 @@
         "@types/node": "*"
       }
     },
+    "node_modules/@types/swagger-jsdoc": {
+      "version": "6.0.4",
+      "resolved": "https://registry.npmjs.org/@types/swagger-jsdoc/-/swagger-jsdoc-6.0.4.tgz",
+      "integrity": "sha512-W+Xw5epcOZrF/AooUM/PccNMSAFOKWZA5dasNyMujTwsBkU74njSJBpvCCJhHAJ95XRMzQrrW844Btu0uoetwQ==",
+      "dev": true,
+      "license": "MIT"
+    },
     "node_modules/@types/yauzl": {
       "version": "2.10.3",
       "resolved": "https://registry.npmjs.org/@types/yauzl/-/yauzl-2.10.3.tgz",