feat(db): m:m interest_berths junction + role flags

Introduces the multi-berth interest model from plan §3.1: a junction
between interests and berths with three role flags so the same berth
can be linked as the primary deal target, an EOI-bundle inclusion,
or a "just exploring" link without conflating semantics.

- 0028 schema migration creates interest_berths with the unique
  partial index "≤1 primary per interest", a unique compound on
  (interest_id, berth_id), and indexes for the public-map "under
  offer" lookup (where is_specific_interest=true).
- Same migration adds desired_length_ft / desired_width_ft /
  desired_draft_ft to interests for the recommender.
- Same migration runs the Phase 2 data migration: every interest
  with a non-null berth_id gets one junction row marked
  is_primary=true, is_specific_interest=true, and is_in_eoi_bundle =
  (eoi_status='signed'). Pre-flight check halts on dangling FKs
  (§14.3 critical case).
- New service src/lib/services/interest-berths.service.ts owns reads
  + writes of the junction. getPrimaryBerth / getPrimaryBerthsForInterests
  feed list pages; upsertInterestBerth demotes the prior primary in
  the same transaction so the unique index is never violated.
- interests.berth_id stays in place this commit so existing callers
  keep working; Phase 2b migrates them onto the helper service and a
  later migration drops the column.

53 dev rows seeded into the junction; tests still green at 996.

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

src/lib/db/schema/relations.ts

@@ -18,7 +18,7 @@ import {
 } from './clients';
 
 // Interests
-import { interests, interestNotes, interestTags } from './interests';
+import { interests, interestNotes, interestTags, interestBerths } from './interests';
 
 // Yachts
 import { yachts, yachtOwnershipHistory, yachtNotes, yachtTags } from './yachts';
@@ -274,6 +274,18 @@ export const interestsRelations = relations(interests, ({ one, many }) => ({
   reminders: many(reminders),
   berthRecommendations: many(berthRecommendations),
   formSubmissions: many(formSubmissions),
+  interestBerths: many(interestBerths),
+}));
+
+export const interestBerthsRelations = relations(interestBerths, ({ one }) => ({
+  interest: one(interests, {
+    fields: [interestBerths.interestId],
+    references: [interests.id],
+  }),
+  berth: one(berths, {
+    fields: [interestBerths.berthId],
+    references: [berths.id],
+  }),
 }));
 
 export const interestNotesRelations = relations(interestNotes, ({ one }) => ({