Security hardening from 2026-07-01 audit

- Exclude supabase/ from Docker build context (leaked service_role key file)
- /api/files: exact per-user namespace match + reject path traversal;
  storage resolveKey rejects ".."/"." segments (fixes cross-user file read)
- Add ownsProperty/Unit/Tenant checks to tenants, maintenance (landlord path),
  and documents (JSON branch, now field-whitelisted) create handlers
- Escape user data in follow-up + payment-link emails (reuse escapeHtml)
- Neutralize CSV formula injection in toCsv + export routes
- Tighter sign-in rate limit (10/min); env-gated email verification + sender
- Per-request nonce CSP; drop script-src 'unsafe-inline' (styles unchanged)
- Add input length bounds; validate follow-ups POST body

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/db/admin-queries.ts

@@ -402,7 +402,11 @@ export async function getUserDetail(id: string) {
 // ── CSV helpers (shared by admin export routes) ─────────────────────────────────
 export function toCsv(headers: string[], rows: (string | number | null | undefined)[][]) {
   const esc = (v: string | number | null | undefined) => {
-    const s = v === null || v === undefined ? "" : String(v)
+    let s = v === null || v === undefined ? "" : String(v)
+    // Neutralize spreadsheet formula injection: values that begin with a
+    // formula trigger (= + - @) or a control char (tab/CR) are prefixed with a
+    // single quote so Excel/Sheets treat them as text, not executable formulas.
+    if (/^[=+\-@\t\r]/.test(s)) s = `'${s}`
     return /[",\n]/.test(s) ? `"${s.replace(/"/g, '""')}"` : s
   }
   return [headers, ...rows].map((r) => r.map(esc).join(",")).join("\n")