feat: add servicelink RPC mesh endpoint

- Add the servicelink submodule and register POST /rpc for node-to-node file operations.
- Require bearer tokens with the mesh scope and apply rate/body-size limits to RPC calls.
- Map database connectivity failures to the existing 504 database error flow, with JSON responses for API routes.
- Cover the new RPC handlers and database error handling with focused pytest tests.
- Bump the NanoShare package version to 1.21.0.

run.py

@@ -13,6 +13,7 @@ from routes import (
   upload_bp,
   health_bp
 )
+from routes.api.link import link_bp as servicelink_bp
 
 # Views for Requests adding the uris
 app.register_blueprint(basic_bp)
@@ -21,6 +22,9 @@ app.register_blueprint(auth_login_bp)
 app.register_blueprint(side_main_bp)
 app.register_blueprint(upload_bp)
 
+# ServiceLink node-to-node mesh endpoint (POST /rpc)
+app.register_blueprint(servicelink_bp)
+
 app.register_blueprint(health_bp, url_prefix='/health')
 
 if __name__ == '__main__':