Schema + migration for the template source table, and TemplateRepository.resolve:
highest active version for (key,channel,locale), scoped override preferred over
the global (scopeId NULL) default.
Migration adds a PARTIAL unique index WHERE scopeId IS NULL so two platform
defaults for the same key can't coexist (Postgres treats NULL as distinct under
a plain UNIQUE — the scoped constraint alone wouldn't catch it). 6 tests.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
