fix: compute totalPages from unpaginated count query

The objects_count query reused the same SELECT that already had
LIMIT/OFFSET applied, so count() always returned at most page_size
rows, making totalPages always 1 regardless of actual row count.

Build a separate count query with only filters and joins (no
pagination) so that totalPages reflects the true number of pages.

Closes #103

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: temrjan

fastapi_jsonapi/data_layers/sqla/orm.py

@@ -425,9 +425,15 @@ async def get_collection(
 
         objects_count = self.default_collection_count
         if not self.disable_collection_count:
+            count_query = self._base_sql.query(
+                model=self.model,
+                filters=filters,
+                jsonapi_join=relationships_info,
+                stmt=self._query,
+            )
             objects_count = await self._base_sql.count(
                 session=self.session,
-                stmt=query,
+                stmt=count_query,
             )
 
         collection = await self.after_get_collection(collection, qs, view_kwargs)