feat(rewards): add CombatRewardService with idempotent victory rewards

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

apps/api/src/rewards/rewards.errors.ts

@@ -0,0 +1,29 @@
+import { HttpException, HttpStatus } from '@nestjs/common';
+
+export type RewardErrorCode = 'COMBAT_NOT_WON' | 'REWARD_STATE_INVALID';
+
+export class RewardDomainError extends HttpException {
+  constructor(
+    public readonly code: RewardErrorCode,
+    status: HttpStatus,
+    message: string,
+  ) {
+    super({ statusCode: status, code, message }, status);
+  }
+}
+
+export function combatNotWon(): RewardDomainError {
+  return new RewardDomainError(
+    'COMBAT_NOT_WON',
+    HttpStatus.CONFLICT,
+    'Only a won combat can grant victory rewards.',
+  );
+}
+
+export function rewardStateInvalid(): RewardDomainError {
+  return new RewardDomainError(
+    'REWARD_STATE_INVALID',
+    HttpStatus.INTERNAL_SERVER_ERROR,
+    'The reward references unavailable data.',
+  );
+}