DATA-2: optional list params + normalize frontend's '==' Gridify operator

- GridifyParams wrapper: nullable page/pageSize/filter/orderBy so list endpoints
  work with no query params (Gridify's non-nullable int Page made [AsParameters]
  treat them as required -> 400). Defaults page=1,pageSize=20.
- Normalize incoming filter '==' -> '=' : the frontend gridify.ts emits '==' for
  equals (its convention) but Gridify's equals is '='. Safe (values are escaped;
  != >= <= =* contain no '=='). Fixes status==Active (frontend default that 500'd).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

GerbilManagerWebAPI/Endpoints/GerbilEndpoints.cs

@@ -15,7 +15,7 @@ namespace GerbilManagerWebAPI.Endpoints
             var group = app.MapGroup("/gerbils").WithTags("Gerbils");
 
             // GET /gerbils  (Gridify: filter/order/page; e.g. status==Active, litterId==…, orderBy=name)
-            group.MapGet("/", async ([AsParameters] GridifyQuery query, ApplicationContext db) =>
+            group.MapGet("/", async ([AsParameters] GridifyParams query, ApplicationContext db) =>
                 TypedResults.Ok(await db.Gerbils.AsNoTracking()
                     .ToPagedResultAsync(query, ToDto)));