feat(checklists): t-paliad-225 Slice B backend — explicit sharing + admin promotion

m/paliad#61 Slice B backend. Implements the explicit-share path
(checklist_shares + visibility predicate extension) and the
global_admin-only promotion / demotion of authored templates to and
from the firm catalog.

Schema (mig 115, idempotent):
- paliad.checklist_shares (uuid id, checklist_id FK, polymorphic
  recipient via xor-check: recipient_kind in {user, office,
  partner_unit, project} with exactly one matching recipient_* column
  populated; granted_by FK; granted_at)
- Hot-path lookup index + per-kind partial UNIQUE indexes prevent
  duplicate grants
- RLS: SELECT owner OR self-recipient (user-kind) OR global_admin;
  INSERT owner-only with granted_by=self; DELETE owner OR global_admin;
  no UPDATE (revoke = DELETE)
- can_see_checklist CREATE OR REPLACE — adds 4 share branches; project-
  share branch uses inline ltree walk over projects.path because
  can_see_project reads auth.uid() (NULL on service-role connection,
  same pattern as visibility.go)
- xor-check verified live: rejects kind='user' with recipient_office
  set; accepts the matching kind/recipient pair

Services:
- ChecklistShareService — Grant (owner-only, validates recipient kind +
  required FK target, friendly 409 on partial-unique-index conflict),
  Revoke (owner or global_admin), ListGrants (owner or global_admin;
  enriches recipient_label via LEFT JOINs)
- ChecklistPromotionService — Promote (global_admin → visibility=global
  + promoted_at/by + audit), Demote (global_admin → target visibility,
  default 'firm', clears promoted_at/by; rejects demote of non-global
  rows)
- ChecklistCatalogService.checklistVisibilityPredicate extended to
  include all 5 share branches; service-role-friendly (no auth.uid())
- ChecklistTemplateService.normaliseSliceAVisibility now accepts
  'shared' as an author-set value; 'global' stays admin-only

Endpoints:
- GET    /api/checklists/templates/{slug}/shares  — list grants (owner/admin)
- POST   /api/checklists/templates/{slug}/shares  — grant
- DELETE /api/checklists/shares/{id}              — revoke
- POST   /api/admin/checklists/{slug}/promote     — promote to global
- POST   /api/admin/checklists/{slug}/demote      — demote (body.target default 'firm')

Audit (paliad.system_audit_log):
- checklist.shared      — recipient_kind + recipient_id in metadata
- checklist.unshared    — same shape, captured pre-DELETE
- checklist.promoted_global — prior_visibility + owner_id
- checklist.demoted     — target_visibility

Tests: validateShareInput covers all 4 kinds (happy + missing-id);
predicate-shape test asserts all 6 visibility branches present;
pqUniqueViolation regex sniff; nullableString helper; SliceB visibility
opens 'shared' but keeps 'global' admin-only.

Hotfix-merge note: head shipped 794617c after Slice A — the
template-edit page route moved from /checklists/{slug}/edit to
/checklists/templates/{slug}/edit to disambiguate from
/checklists/instances/{id}. Slice B routes follow the safe
/<resource>/<noun>/{id} pattern (no new {slug}-then-verb endpoints).

cmd/server/main.go

@@ -152,8 +152,10 @@ func main() {
 		eventTypeSvc := services.NewEventTypeService(pool, users)
 		deadlineSvc := services.NewDeadlineService(pool, projectSvc, eventTypeSvc)
 		// t-paliad-225 Slice A — user-authored checklist templates.
+		// Slice B adds checklist_shares grants + admin promotion.
 		checklistCatalogSvc := services.NewChecklistCatalogService(pool)
 		sysAuditSvc := services.NewSystemAuditLogService(pool)
+		checklistTemplateSvc := services.NewChecklistTemplateService(pool, checklistCatalogSvc, sysAuditSvc, users)
 		svcBundle = &handlers.Services{
 			Project:        projectSvc,
 			Team:           teamSvc,
@@ -184,7 +186,9 @@ func main() {
 			Note:           services.NewNoteService(pool, projectSvc, appointmentSvc),
 			ChecklistInst:  services.NewChecklistInstanceService(pool, projectSvc, checklistCatalogSvc),
 			ChecklistCatalog: checklistCatalogSvc,
-			ChecklistTemplate: services.NewChecklistTemplateService(pool, checklistCatalogSvc, sysAuditSvc, users),
+			ChecklistTemplate: checklistTemplateSvc,
+			ChecklistShare: services.NewChecklistShareService(pool, checklistTemplateSvc, sysAuditSvc, users),
+			ChecklistPromotion: services.NewChecklistPromotionService(pool, checklistTemplateSvc, sysAuditSvc, users),
 			Mail:           mailSvc,
 			Invite:         inviteSvc,
 			Agenda:         services.NewAgendaService(pool, users, eventTypeSvc),