feat: OpenAPI 3.0 spec with Swagger UI

- GET /openapi.yaml: raw OpenAPI spec
- GET /openapi.json: JSON-formatted spec
- GET /docs: Swagger UI for interactive API docs
- 19 endpoints documented with schemas
- Added pyyaml dependency

pyproject.toml

@@ -10,6 +10,7 @@ dependencies = [
     "flask-cors>=4.0",
     "gunicorn>=21.0",
     "python-dotenv>=1.0",
+    "pyyaml>=6.0",
 ]
 
 [project.optional-dependencies]