fix(admin): keyboard-reachable audit payload toggle; drop dead step ternary

Review follow-ups: the audit row's expand was mouse-only (onclick on a bare
<tr>). Move the toggle to a real <button> in the actions cell with
aria-expanded + aria-label so keyboard/AT users can open a row's payload; the
row onclick stays as a mouse convenience (stopPropagation avoids a double
toggle). Also simplify a no-op `step={'%' ? '1' : '1'}` to step="1" on the
threshold inputs.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/admin/HealthChecks.svelte

@@ -158,7 +158,7 @@
                             <span>{f.label}{f.unit ? ` (${f.unit})` : ''}</span>
                             <input
                                 type="number"
-                                step={f.unit === '%' ? '1' : '1'}
+                                step="1"
                                 min="0"
                                 bind:value={form[f.key]}
                                 data-testid={`threshold-${f.key}`}