Add hybrid admin delete for quest definitions

Owner decision 2026-09-26: "delete if there's no history,
retire/deactivate if there is". A definition with no completion event
on any instance is hard-deleted (instances, assignees, windows, the row
and its unreferenced rule) in one transaction under the connection
lock; one with history is retired via set_quest_definition_active and
everything is preserved (D-005).

- core: QuestDefinitionsDao.delete_if_no_history (atomic history check +
  deletes) and quest_definitions.delete_quest_definition
- api: DELETE /api/v1/admin/quest-definitions/{id} on the admin router
- admin: Delete task control with a hybrid-explaining confirm in the
  edit sheet; the list removes or shows the row inactive
- docs: ADMIN-SPEC Definitions section

Task 3efb07b4-7bfa-45af-b36a-57d81249a39d

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

design/ADMIN-SPEC.md

@@ -64,7 +64,8 @@ Header `Tasks` / `7 definitions · 3 children` + gradient `New` button. Filter c
 (All / one per child): active `#18181D` fill white text, inactive white with border.
 Rows: glyph 17px · title 13.5px/700 · meta 11px/600 `<assignees> · <recurrence> · <windows>`
 · `chevron-right`. An inactive definition renders the whole row at
-`--nq-a-ink-tertiary` with `· inactive` in the meta — never hard-deleted (glossary).
+`--nq-a-ink-tertiary` with `· inactive` in the meta. A definition with completion
+history is never hard-deleted (glossary, D-017): Delete retires it to this inactive state.
 The row being edited takes `--nq-a-selected` with brand-blue glyph, `Editing` meta and
 `chevron-down`.
 
@@ -83,6 +84,17 @@ Bottom sheet, `--nq-a-radius-sheet`, `--nq-a-sheet-shadow`, 38×4px grabber, pad
 Footer: `Cancel` (flex 1, bordered) + `Save changes` (flex 2, gradient), both 13px vertical
 padding. Sheet enters over the dimmed list in `--nq-dur-modal`.
 
+Delete (editing an existing definition only, never on `New`): a full-width bordered
+`Delete task` button below the footer, danger-ink text. Tapping it swaps the footer for a
+danger-tinted confirm — "Delete this task? If it has no completion history it is deleted;
+otherwise it is retired (deactivated) and its history is kept." — with `Go back` and a
+danger-filled `Delete task`. Confirming calls `DELETE /api/v1/admin/quest-definitions/{id}`
+(admin plane only; the kids' panel has no equivalent control). Hybrid outcome (D-017): no
+completion event on any instance → the definition, its instances, assignees, windows and
+unreferenced rule are hard-deleted and the row leaves the list; otherwise nothing is
+deleted, the definition is deactivated and the row stays, shown inactive. An error keeps
+the sheet open with the API's detail.
+
 ### 3.3 Icon picker
 Each definition carries one optional icon, shown on both surfaces. The picker offers three
 sources: