fix(audit): non-Documenso backlog sweep — port-binding, NULLS NOT DISTINCT, custom merge tokens, company docs

Wave through the remaining audit-final-deferred items that aren't blocked
on the back-burnered Documenso work.

Multi-tenant isolation:
- Storage proxy ProxyTokenPayload gains optional `p` (port slug) claim;
  verifier asserts `key.startsWith(${p}/)`. Defense-in-depth against a
  buggy issuer in some future code path that mixes port scopes — every
  storage key generated by generateStorageKey() already prefixes the
  slug. document-sends opts in for 24h emailed download links; other
  callers continue working unchanged via the optional field.

DB schema reconciliation:
- Migration 0047 rebuilds system_settings unique index with NULLS NOT
  DISTINCT (Postgres 15+) so global settings (port_id IS NULL) are
  uniquely keyed by `key` alone. Surfaced + dedupe'd 65 duplicate
  (storage_backend, NULL) rows that had accumulated from race-prone
  delete-then-insert patterns in ocr-config / settings / residential-
  stages / ai-budget services. All four services converted to true
  onConflictDoUpdate upserts so the race window is closed.

API uniformity:
- Response shape standardization: 16 routes converted from
  `{ success: true }` to 204 No Content. CLAUDE.md documents the
  convention (`{ data: <T> }` for content, 204 for empty mutations,
  portal-auth retains `{ success: true }` for the frontend's auth chain).
- req.json() → parseBody() migration across 9 admin/CRM routes
  (custom-fields, expenses/export ×3, currency convert,
  search/recently-viewed, admin/duplicates, berths/pdf-{upload-url,
  versions, parse-results}). Uniform 400 error shapes for
  ZodError-flagged bodies.

Custom-fields merge tokens (shipped end-to-end):
- merge-fields.ts gains CUSTOM_MERGE_TOKEN_RE + helpers for the
  `{{custom.<fieldName>}}` shape.
- document-templates validator accepts the dynamic shape alongside
  the static catalog tokens.
- document-sends.service mergeCustomFieldValues resolver fetches
  per-port custom_field_definitions for client/interest/berth contexts
  and substitutes stored values keyed by `{{custom.fieldName}}`.
- custom-fields-manager amber banner updated to reflect that merge
  tokens now expand (search index + entity-diff remain documented
  design limitations).

/api/v1/files cross-entity filtering:
- Validator + listFiles + uploadFile accept companyId AND yachtId
  alongside clientId. file-upload-zone propagates both.
- New CompanyFilesTab component mirrors ClientFilesTab; restored as a
  visible Documents tab in company-tabs.tsx (was a hidden stub).

Inline TODOs:
- Reviewed remaining two TODOs (per-user reminder schedule, import
  worker handlers). Both are placeholders for future feature surfaces,
  not bugs — per-port digest works for every customer; nothing
  currently enqueues import jobs (verified). Annotated in BACKLOG.

BACKLOG.md updated to reflect what landed and what's still pending
(Documenso-related items still bundled with the back-burnered phases).

Tests: 1185/1185 vitest, tsc clean.

src/app/api/v1/admin/duplicates/handlers.ts

@@ -1,7 +1,9 @@
-import { NextResponse } from 'next/server';
+import { NextRequest, NextResponse } from 'next/server';
+import { z } from 'zod';
 import { and, eq, inArray } from 'drizzle-orm';
 
 import type { AuthContext } from '@/lib/api/helpers';
+import { parseBody } from '@/lib/api/route-helpers';
 import { db } from '@/lib/db';
 import { clients, clientMergeCandidates } from '@/lib/db/schema/clients';
 import { errorResponse, NotFoundError, ValidationError } from '@/lib/errors';
@@ -11,6 +13,11 @@ import {
   type MergeFieldChoices,
 } from '@/lib/services/client-merge.service';
 
+const confirmMergeSchema = z.object({
+  winnerId: z.string().min(1),
+  fieldChoices: z.record(z.string(), z.string()).optional(),
+});
+
 /**
  * GET /api/v1/admin/duplicates
  *
@@ -70,19 +77,13 @@ export async function listHandler(_req: Request, ctx: AuthContext): Promise<Next
  * service which is the only path that touches client_merge_log.
  */
 export async function confirmMergeHandler(
-  req: Request,
+  req: NextRequest,
   ctx: AuthContext,
   params: { id?: string },
 ): Promise<NextResponse> {
   try {
     const id = params.id ?? '';
-    const body = (await req.json().catch(() => ({}))) as {
-      winnerId?: string;
-      fieldChoices?: MergeFieldChoices;
-    };
-    if (!body.winnerId) {
-      throw new ValidationError('winnerId is required');
-    }
+    const body = await parseBody(req, confirmMergeSchema);
 
     const [candidate] = await db
       .select()
@@ -111,7 +112,7 @@ export async function confirmMergeHandler(
       loserId,
       mergedBy: ctx.userId,
       callerPortId: ctx.portId,
-      fieldChoices: body.fieldChoices,
+      fieldChoices: body.fieldChoices as MergeFieldChoices | undefined,
     });
 
     return NextResponse.json({ data: result });