feat(api): add Patch<T> enum for JSON merge-patch semantics

Introduce a three-state Patch<T> (Unchanged / Remove / Set) and a
serde helper patch_field that distinguishes an omitted JSON field
from an explicit null. Wired up next as the field type for the
removable settings on PATCH /v1/users/{user}.

src/api/mod.rs

@@ -28,6 +28,7 @@ mod config_store;
 mod events;
 mod http_utils;
 mod model;
+mod patch;
 mod runtime_edge;
 mod runtime_init;
 mod runtime_min;