ci(swagger): add OpenAPI validation job to CI

- Install @stoplight/spectral-cli as dev dependency
- Add "Swagger Validation & Coverage" job to .gitea/workflows/ci.yml
- Run spectral lint on server/openapi.yaml
- Run npm test to execute coverage tests
- Fail CI if spec is invalid or coverage is incomplete
- Runs on every push/PR alongside existing jobs

package.json

@@ -29,6 +29,7 @@
     "yamljs": "^0.3.0"
   },
   "devDependencies": {
+    "@stoplight/spectral-cli": "^6.16.0",
     "@vitest/coverage-v8": "^4.1.6",
     "concurrently": "^7.6.0",
     "nock": "^14.0.15",