feat: add trip preference overrides with precedence resolution

backend/libs/trips/src/preference-precedence.spec.ts

@@ -0,0 +1,47 @@
+import { resolveEffectivePreference } from './preference-precedence';
+import type { UserPreferenceFields } from '../../users/src';
+
+describe('resolveEffectivePreference', () => {
+  const appDefault: UserPreferenceFields = {
+    preferredPace: 'moderate',
+    preferredBudgetLevel: 'medium',
+    maxWalkingDistanceKm: 5,
+    preferredStartTime: null,
+    childFriendlyPreferred: false,
+    interests: [],
+    notes: null,
+  };
+
+  it('falls back to the application default when nothing is set', () => {
+    expect(
+      resolveEffectivePreference(undefined, undefined, appDefault),
+    ).toEqual(appDefault);
+  });
+
+  it('prefers the persistent user preference over the application default', () => {
+    const userPref = { ...appDefault, preferredPace: 'relaxed' };
+    expect(
+      resolveEffectivePreference(undefined, userPref, appDefault).preferredPace,
+    ).toBe('relaxed');
+  });
+
+  it('prefers a trip-specific override over the persistent user preference', () => {
+    const userPref = { ...appDefault, preferredPace: 'relaxed' };
+    const override = { preferredPace: 'fast' };
+    expect(
+      resolveEffectivePreference(override, userPref, appDefault).preferredPace,
+    ).toBe('fast');
+  });
+
+  it('merges field-by-field rather than replacing the whole object', () => {
+    const userPref = {
+      ...appDefault,
+      preferredPace: 'relaxed',
+      childFriendlyPreferred: true,
+    };
+    const override = { preferredPace: 'fast' };
+    const result = resolveEffectivePreference(override, userPref, appDefault);
+    expect(result.preferredPace).toBe('fast');
+    expect(result.childFriendlyPreferred).toBe(true);
+  });
+});