feat: add utoipa path annotations to all API handlers and wire OpenAPI spec

- Add #[utoipa::path] annotations to all 70+ handler functions across
  auth, config, workflow, and message modules
- Add IntoParams/ToSchema derives to Pagination, PaginatedResponse, ApiResponse
  in erp-core, and MessageQuery/TemplateQuery in erp-message
- Collect all module paths into OpenAPI spec via AuthApiDoc, ConfigApiDoc,
  WorkflowApiDoc, MessageApiDoc structs in erp-server main.rs
- Update openapi_spec handler to merge all module specs
- The /docs/openapi.json endpoint now returns complete API documentation
  with all endpoints, request/response schemas, and security requirements

crates/erp-core/Cargo.toml

@@ -15,3 +15,4 @@ tracing.workspace = true
 axum.workspace = true
 sea-orm.workspace = true
 async-trait.workspace = true
+utoipa.workspace = true