feat(interests): wire yachtId, enforce ownership + stage-gate

- Add yachtId (optional) to createInterestSchema + listInterestsSchema
  (updateInterestSchema inherits it via partial() automatically).
- Add assertYachtBelongsToClient helper that accepts direct client
  ownership OR company-represented clients with an active membership
  in the owning company.
- createInterest + updateInterest validate yacht ownership whenever
  yachtId is supplied/changed.
- changeInterestStage rejects moving out of stage=open with yachtId
  null (ValidationError).
- listInterests filter supports yachtId.
- Integration tests cover all 7 paths; validator test for yachtId.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/unit/validators.test.ts

@@ -131,6 +131,14 @@ describe('createInterestSchema', () => {
     const result = createInterestSchema.safeParse({ clientId: 'c1', reminderDays: 0 });
     expect(result.success).toBe(false);
   });
+
+  it('createInterestSchema accepts yachtId', () => {
+    const result = createInterestSchema.safeParse({
+      clientId: 'c1',
+      yachtId: 'y1',
+    });
+    expect(result.success).toBe(true);
+  });
 });
 
 describe('changeStageSchema', () => {