/firm sales export <firm> <days> — explorer deep link / signed CSV (not Hastebin)

Part of PAR-175 (firm sales commands). Parity with legacy /pb sales export <firm> <days>, modernised.

Legacy exported N days of sales to a Hastebin paste and returned the URL (cap max-sales-export-days: 30). Replace the Hastebin dependency:

• Preferred: return a deep link into the economy-explorer per-firm page (verified routes): /firms/[firmName] already renders a ChestShop sales summary card that links to the full drill-down at /chestshop/firms/{firmId}. Link there (optionally with a window query param). It's gated by the explorer's canSeeFinancials (FINANCIAL/ADMIN employees + staff), so auth is already handled — no need to re-implement it. (Note: /me/market is the personal page — that belongs to PAR-181, not the firm export.) A dedicated /firms/{firmId}/sales?days=N route would be new if a window-scoped page is wanted.
• Or: a signed/expiring CSV download via treasury-rest-api (authenticated), if a raw spreadsheet file is wanted — backed by the PAR-176 read API.

Keep the configurable day cap (max-sales-export-days) and the "export limit" message. Access: owner/admin (mirror legacy playerbusinesses.sales.export).

Depends on: PAR-176 read API (for the CSV path) and/or the existing explorer firm pages.