Add Swagger docs at /docs and /openapi.json with split OpenAPI spec

Made-with: Cursor

backend/package.json

@@ -10,6 +10,7 @@
     "migrate": "tsx src/db/migrate.ts"
   },
   "dependencies": {
+    "@apidevtools/swagger-parser": "^12.1.0",
     "better-sqlite3": "^11.6.0",
     "cors": "^2.8.5",
     "dotenv": "^16.4.5",
@@ -17,6 +18,7 @@
     "express-rate-limit": "^7.4.1",
     "nostr-tools": "^2.4.4",
     "pg": "^8.13.1",
+    "swagger-ui-express": "^5.0.1",
     "uuid": "^10.0.0"
   },
   "devDependencies": {
@@ -25,6 +27,7 @@
     "@types/express": "^4.17.21",
     "@types/node": "^22.9.0",
     "@types/pg": "^8.11.10",
+    "@types/swagger-ui-express": "^4.1.8",
     "@types/uuid": "^10.0.0",
     "tsx": "^4.19.2",
     "typescript": "^5.6.3"