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>

src/lib/validators/interests.ts

@@ -7,6 +7,7 @@ import { PIPELINE_STAGES, LEAD_CATEGORIES } from '@/lib/constants';
 
 export const createInterestSchema = z.object({
   clientId: z.string().min(1),
+  yachtId: z.string().optional(),
   berthId: z.string().optional(),
   pipelineStage: z.enum(PIPELINE_STAGES).default('open'),
   leadCategory: z.enum(LEAD_CATEGORIES).optional(),
@@ -34,6 +35,7 @@ export const changeStageSchema = z.object({
 
 export const listInterestsSchema = baseListQuerySchema.extend({
   clientId: z.string().optional(),
+  yachtId: z.string().optional(),
   berthId: z.string().optional(),
   pipelineStage: z
     .string()