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/lib/templates/merge-fields.ts

@@ -91,3 +91,25 @@ export const MERGE_FIELDS: MergeFieldCatalog = {
 export const VALID_MERGE_TOKENS: ReadonlySet<string> = new Set(
   Object.values(MERGE_FIELDS).flatMap((scope) => scope.map((field) => field.token)),
 );
+
+/**
+ * Custom-field merge tokens follow the pattern `{{custom.<fieldName>}}`
+ * where fieldName matches the per-port custom field's `fieldName` (the
+ * machine identifier, not the display label). Field names are validated
+ * at definition time as `[a-z][a-z0-9_]*` so this regex is the matching
+ * recogniser. Tokens that match this shape bypass the static
+ * `VALID_MERGE_TOKENS` check — the resolver fetches the actual
+ * definitions per port at expand time.
+ */
+export const CUSTOM_MERGE_TOKEN_RE = /^\{\{custom\.[a-z][a-z0-9_]*\}\}$/;
+
+/** True when the token is a `{{custom.<fieldName>}}` shape. */
+export function isCustomMergeToken(token: string): boolean {
+  return CUSTOM_MERGE_TOKEN_RE.test(token);
+}
+
+/** Extract the fieldName from a `{{custom.<fieldName>}}` token. */
+export function extractCustomFieldName(token: string): string | null {
+  const match = token.match(/^\{\{custom\.([a-z][a-z0-9_]*)\}\}$/);
+  return match?.[1] ?? null;
+}