feat(intake): act on a signed link from the notification (#224)

Mounted publicly, deliberately not behind requireAdminGate. These are clicked from an inbox by someone who is not signed in, which is the whole point; the signature is what protects them.

GET confirms and changes nothing, POST acts. Mail scanners and corporate link-rewriting gateways issue a GET against every URL in a message before a human sees it, so a GET that discarded a draft would fire itself on delivery — carrying a valid signature, looking entirely legitimate in the log, and nobody would know to go and recover it. That is the case the split exists for and it has its own test.

Forged, replayed, upgraded and expired links are each refused with the same 403. Distinguishing them would tell somebody probing which of those they had achieved. There is no signable publish, and asking for one finds no handler.

The two registry guard tests are updated rather than worked around: they assert the full set of settings and template keys, so adding either is exactly what should trip them.

Backend now 367 unit and 329 integration, all passing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/tests/integration/adminSettings.integration.test.ts

@@ -30,7 +30,10 @@ describe('GET /api/admin/settings', () => {
       passwordResetHours: 1,
       greetingFormat: 'Hi {{firstName}},',
       greetingFallback: 'Hi,',
-      draftingModel: 'claude-sonnet-5'
+      draftingModel: 'claude-sonnet-5',
+      // Empty by default: nowhere to send the intake notification is a working
+      // configuration, and means simply do not send one (#224).
+      intakeNotifyEmail: ''
     });
   });
 });