Add lookup tables, payment methods, tax exemptions, and processor link APIs

Replace unit_status and item_condition pgEnums with company-scoped lookup
tables that support custom values. Add account_payment_method table,
tax_exemption table with approve/revoke workflow, and CRUD routes for
processor links. Validate inventory unit status/condition against lookup
tables at service layer.

packages/backend/src/db/migrations/meta/_journal.json

@@ -50,6 +50,13 @@
       "when": 1774653924179,
       "tag": "0006_add_consignment",
       "breakpoints": true
+    },
+    {
+      "idx": 7,
+      "version": "7",
+      "when": 1774662300000,
+      "tag": "0007_accounts_lookups",
+      "breakpoints": true
     }
   ]
 }