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]/watchers/route.ts

@@ -0,0 +1,39 @@
+import { NextResponse } from 'next/server';
+import { z } from 'zod';
+
+import { withAuth, withPermission } from '@/lib/api/helpers';
+import { parseBody } from '@/lib/api/route-helpers';
+import { errorResponse } from '@/lib/errors';
+import { addDocumentWatcher, listDocumentWatchers } from '@/lib/services/documents.service';
+
+const addWatcherSchema = z.object({
+  userId: z.string().min(1),
+});
+
+export const GET = withAuth(
+  withPermission('documents', 'view', async (_req, ctx, params) => {
+    try {
+      const watchers = await listDocumentWatchers(params.id!, ctx.portId);
+      return NextResponse.json({ data: watchers });
+    } catch (error) {
+      return errorResponse(error);
+    }
+  }),
+);
+
+export const POST = withAuth(
+  withPermission('documents', 'edit', async (req, ctx, params) => {
+    try {
+      const body = await parseBody(req, addWatcherSchema);
+      const watcher = await addDocumentWatcher(params.id!, ctx.portId, body.userId, {
+        userId: ctx.userId,
+        portId: ctx.portId,
+        ipAddress: ctx.ipAddress,
+        userAgent: ctx.userAgent,
+      });
+      return NextResponse.json({ data: watcher }, { status: 201 });
+    } catch (error) {
+      return errorResponse(error);
+    }
+  }),
+);