feat(documents): detail page with signers, watchers, activity, actions

Replaces the PR4 stub at /documents/[id] with the full Phase A detail
view: gradient header strip, status-aware action bar (Cancel /
Download / Email signatories), per-signer remind + copy-link, watcher
list with remove, and activity timeline. Adds the supporting endpoints
(cancel, compose-completion-email, watchers GET/POST/DELETE) and
listDocumentWatchers / addDocumentWatcher / removeDocumentWatcher
service helpers. The document GET now serves the aggregator shape
when ?detail=true.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/app/api/v1/documents/[id]/route.ts

@@ -5,6 +5,7 @@ import { parseBody } from '@/lib/api/route-helpers';
 import { errorResponse } from '@/lib/errors';
 import {
   getDocumentById,
+  getDocumentDetail,
   updateDocument,
   deleteDocument,
 } from '@/lib/services/documents.service';
@@ -13,6 +14,11 @@ import { updateDocumentSchema } from '@/lib/validators/documents';
 export const GET = withAuth(
   withPermission('documents', 'view', async (req, ctx, params) => {
     try {
+      const url = new URL(req.url);
+      if (url.searchParams.get('detail') === 'true') {
+        const detail = await getDocumentDetail(params.id!, ctx.portId);
+        return NextResponse.json({ data: detail });
+      }
       const doc = await getDocumentById(params.id!, ctx.portId);
       return NextResponse.json({ data: doc });
     } catch (error) {