Add endpoint for paying an invoice so that users don't get expired qr codes

backend/spec/api.md

@@ -219,7 +219,7 @@ Attempts to pay a new subscription invoice (Stripe is pay-in-advance, so this fi
 
 1. **NWC auto-pay**: if the tenant has a `nwc_url`, run `billing.pay_invoice_nwc`. On success, done. On failure, record the error via `command.set_tenant_nwc_error`, log it, summarize it for the eventual DM, and fall through.
 2. **Card on file**: if `stripe.has_payment_method`, do nothing — Stripe charges automatically for this attempt.
-3. **Manual payment**: send a DM via `robot.send_dm` telling the tenant payment is due (including the summarized NWC error, if any), with a link to the app for manual Lightning payment.
+3. **Manual payment**: send a DM via `robot.send_dm` telling the tenant payment is due (including the summarized NWC error, if any). The DM includes a link to the dashboard payment view for this invoice — `{env.app_url}/account?invoice={stripe_invoice_id}` — where the tenant can review the invoice and pay by Lightning or card.
 
 ## `invoice.paid`
 

backend/spec/env.md

@@ -8,6 +8,7 @@ Members (all populated from environment variables):
 - `server_port: u16` - from `SERVER_PORT`
 - `server_admin_pubkeys: Vec<String>` - admin pubkeys from `SERVER_ADMIN_PUBKEYS`
 - `server_allow_origins: Vec<String>` - CORS origins from `SERVER_ALLOW_ORIGINS`
+- `app_url: String` - public base URL of the frontend app from `APP_URL`, with any trailing slash stripped; used to build tenant-facing links such as the invoice payment link in billing DMs
 - `database_url: String` - from `DATABASE_URL`
 - `robot_name: String` - from `ROBOT_NAME`
 - `robot_wallet: String` - the system NWC URL from `ROBOT_WALLET`, used to issue/look up bolt11 invoices