feat(checklists): t-paliad-225 Slice C backend — template versioning + catalog Version

m/paliad#61 Slice C backend.

Schema (mig 116, idempotent):
- ALTER paliad.checklists ADD COLUMN version int NOT NULL DEFAULT 1.
  Pre-Slice-C rows default to 1 (the column was added with DEFAULT
  so the UPDATE clause is a no-op safety net).
- ALTER paliad.checklist_instances ADD COLUMN template_version int.
  NULL on existing rows — instance detail page leaves the "outdated"
  badge off when the snapshot version is unknown.

Services:
- ChecklistTemplateService.Update — version bumps on title/body
  changes (the meaningful edits that warrant notifying instance
  owners). Pure metadata tweaks (description/court/reference/deadline)
  update updated_at without bumping. Emits the new 'checklist.versioned'
  audit event with prior_version + new_version metadata.
- ChecklistInstanceService.Create — captures snapshot_version
  alongside the body snapshot.
- ChecklistCatalogService — CatalogEntry grew a Version field
  (1 for static; live column for authored). ListVisible / Find
  populate it.
- Models — Checklist.Version int; ChecklistInstance.TemplateVersion *int.
- /api/checklists/{slug} response now includes version so the
  instance detail page can compare against the snapshot.

Migration verified live via BEGIN..ROLLBACK against paliad.checklists
and paliad.checklist_instances.

Build hygiene: go build/vet/test ./internal/... + TestBootSmoke
./cmd/server/ all green.

internal/db/migrations/116_checklist_versioning.down.sql

@@ -0,0 +1,7 @@
+-- Reverse of mig 116 — t-paliad-225 / m/paliad#61 Slice C.
+
+ALTER TABLE paliad.checklist_instances
+    DROP COLUMN IF EXISTS template_version;
+
+ALTER TABLE paliad.checklists
+    DROP COLUMN IF EXISTS version;