fix(audit-tier-4): tenant-isolation defense-in-depth

Closes the audit's HIGH §10 + MED §§17–22 isolation footguns. None of
these are user-impactful TODAY — every site is preceded by a port-
scoped read or pre-validated by ctx.portId — but each is a future-
refactor accident waiting to happen, so the SQL itself now pins the
tenant boundary:

* mergeClients gains a callerPortId option; the route caller passes
  ctx.portId.  removeInterestBerth now requires portId and verifies
  both the interest and the berth share it before deleting the
  junction row.  All three callers updated.
* Six service mutations now scope the WHERE to (id, portId):
  form-templates update + delete, invoices.detectOverdue per-row
  update, notifications.markRead, clients.deleteRelationship.
  company-memberships uses an inArray sub-select against port
  companies (no port_id column on the table itself), covering
  updateMembership / endMembership / setPrimary.
* Port-scoped file lookups in portal.getDocumentDownloadUrl,
  reports.getDownloadUrl (file presign), berth-reservations.activate
  (contractFileId attach guard), and residential.getResidentialInterestById
  (residentialClient join).

Test status: 1168/1168 vitest, tsc clean.

Refs: docs/audit-comprehensive-2026-05-05.md HIGH §10 + MED §§17–22
(auditor-B3 Issues 1–5,7).

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

src/lib/services/portal.service.ts

@@ -301,8 +301,13 @@ export async function getDocumentDownloadUrl(
   const fileId = doc.signedFileId ?? doc.fileId;
   if (!fileId) return null;
 
+  // Defense-in-depth: scope the file lookup to the same port. Today the
+  // doc → file relationship is tightly coupled (signedFileId/fileId are
+  // assigned in same-port flows), but a future Documenso-webhook bug or
+  // bulk-import drift could leave a foreign-port file id on a doc and
+  // this presign call would otherwise return a URL to those bytes.
   const file = await db.query.files.findFirst({
-    where: eq(files.id, fileId),
+    where: and(eq(files.id, fileId), eq(files.portId, portId)),
   });
 
   if (!file) return null;