Multi-chapter affiliation data model

Schema design: `user_chapter_affiliations(id UUID PK, user_id UUID FK users.id, chapter_id UUID FK chapters.id, role_in_chapter TEXT, is_primary BOOLEAN DEFAULT false, joined_at TIMESTAMPTZ, created_at TIMESTAMPTZ DEFAULT now())`. Enforce max-5 with a trigger: `CREATE TRIGGER enforce_max_chapter_affiliations BEFORE INSERT ON user_chapter_affiliations FOR EACH ROW EXECUTE FUNCTION check_max_affiliations()`. For the PostgREST query, use Supabase's embedded resource syntax: `.select('*, chapter_affiliations:user_chapter_affiliations(*, chapter:chapters(id, name, region))')`. The `ChapterAffiliation` Dart model should include `chapterName` and `regionName` for display — avoid separate lookups.

NHF's duplicate-reporting concern (section 3.2 of likeperson.md) means the UI layer must display all chapter memberships prominently in the user detail screen so coordinators can see which other chapters claim the user. The data model here is the foundation for that display.

Unit tests: (1) ChapterAffiliation.fromJson() correctly parses all fields including nested chapter name; (2) AppUser.fromJson() with embedded affiliations array deserialises to correct List; (3) adding a 6th affiliation via repository throws MaxChapterAffiliationsExceeded. Integration tests against local Supabase: (1) seed a user with 3 affiliations, fetchUsers returns user with exactly 3 affiliations in list; (2) verify N+1 query prevention — fetching 20 users with affiliations must produce exactly 1 PostgREST request (check via Supabase logs or network interceptor); (3) attempt to insert a 6th affiliation via direct SQL and confirm constraint violation; (4) confirm RLS prevents org_b admin from reading org_a user's affiliations; (5) confirm primary affiliation partial unique index rejects a second is_primary=true row for the same user.