revert: restore preset field name (Backend reverted breaking change)

Backend Brooke reverted the breaking change from BUILD:347. The generate
endpoint continues to use "preset" field name, not "pattern_type".

This reverts commit 834e1a8 and restores the original API contract.

Changes:
- Reverted GenerateSlotDefinitionsRequest: pattern_type → preset
- Reverted GenerateSlotDefinitionsModal to send preset in request body

Backend BUILD:348 has restored the original "preset" field.

src/app/[locale]/admin/clubs/[club_id]/slot-definitions/GenerateSlotDefinitionsModal.tsx

@@ -113,7 +113,7 @@ export default function GenerateSlotDefinitionsModal({
     setError(null);
 
     const request: GenerateSlotDefinitionsRequest = {
-      pattern_type: preset,
+      preset,
       court_ids: selectedCourtIds,
       pattern_overrides: buildPatternOverrides(),
     };

src/types/slot-definitions.ts

@@ -147,7 +147,7 @@ export interface PatternOverrides {
  * Generate endpoint request
  */
 export interface GenerateSlotDefinitionsRequest {
-  pattern_type: SlotDefinitionPreset;
+  preset: SlotDefinitionPreset;
   court_ids: number[];
   pattern_overrides?: PatternOverrides;
   valid_from?: string; // YYYY-MM-DD