Reshape the triggers table to allow a GROUP owner, mirroring
0050_group_scripts exactly. A group-owned trigger is a TEMPLATE: never
dispatched directly, but unioned into every descendant app's match
queries via the ancestor-chain CTE (live, no per-app materialization).
- 0056_group_triggers.sql: add nullable group_id (FK→groups ON DELETE
RESTRICT), make app_id nullable, add the exactly-one owner CHECK, and
split the name-unique + dispatch-hot indexes into per-owner partials
(idx_triggers_group_kind_enabled serves the chain-union lookups).
- Detail tables unchanged (they hang off trigger_id).
Schema snapshot blessed (56 migrations); existing trigger tests green.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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