feat(templates): allow deleting custom event templates

[mindsers]

Summary

Adds the ability to delete custom event templates from the template view page. Previously templates could only be created and updated, never removed.

Behavior

• Delete button on the template view page (/settings/congregation/templates/:id), manager-only.
• System templates refused — day-off and freeform are looked up by key at runtime (createDayOff / createFreeformEvent), so they can never be deleted. The button is hidden for them and the server refuses the intent.
• In-use templates refused — Event.templateId is ON DELETE SET NULL. Deleting a template still linked to events would orphan those events (templateId → NULL), and key-filtered queries silently drop null-template rows — the exact class of bug the 20260720* migration series had to repair. The server blocks deletion and surfaces the event count; the button is disabled while events reference the template.
• Cascade FKs (parts, serviceParts, responsibles, allowed-role rows) handle child cleanup, so a single delete suffices.

Implementation

• deleteTemplate(db, templateId, congregationId) returns a discriminated result (ok / not-found / system / in-use).
• view.tsx action branches on intent (delete vs default duplicate); loader exposes canDelete + eventCount; UI adds a destructive button + InlineDeleteDialog.
• New i18n keys (en/fr) for success / error / in-use messages.
• Extracted duplicateTemplate into its own duplicate-template.server.ts module (+ co-located test) to keep event-templates.server.ts under the file-size budget.
• Drive-by fix: missing React key on a bare fragment in the template edit parts table.

Testing

• TDD: 5 new unit tests for deleteTemplate covering success, not-found, system-refusal, in-use-refusal (asserts no delete + reports count), and congregation scoping.
• Full events suite green; typecheck, lint, file-size, and service-test-coverage gates all pass.