feat: add trip preference overrides with precedence resolution

backend/libs/trips/src/trip-preference-overrides.service.ts

@@ -0,0 +1,24 @@
+import { Injectable } from '@nestjs/common';
+import { TripPreferenceOverridesRepository } from './trip-preference-overrides.repository';
+import type { PreferenceSubject, TripPreferenceOverride } from './trip.types';
+
+@Injectable()
+export class TripPreferenceOverridesService {
+  constructor(private readonly repository: TripPreferenceOverridesRepository) {}
+
+  listOverrides(tripId: string): Promise<TripPreferenceOverride[]> {
+    return this.repository.listByTrip(tripId);
+  }
+
+  upsertOverride(
+    tripId: string,
+    subject: PreferenceSubject,
+    overrides: Record<string, unknown>,
+  ): Promise<TripPreferenceOverride> {
+    return this.repository.upsert(tripId, subject, overrides);
+  }
+
+  removeOverride(tripId: string, overrideId: string): Promise<void> {
+    return this.repository.remove(tripId, overrideId);
+  }
+}