Discount and invoice-line wire-shape gaps (metadata, total_discount_amounts, expand, discount events, resume invoices) #21

What happens

While working on #17, we found several gaps in the shapes the API returns. They predate #17 and don't change any totals.

1. Plan-line metadata. Plan lines always return metadata: {}. Stripe copies the subscription's metadata onto the plan line (probed through the Stripe MCP, 2026-10-16). The line builder is in billing/invoicing.py (~490).
2. total_discount_amounts. compute_totals leaves zero-amount discounts out of the invoice's total_discount_amounts. Lines now list {amount: 0, discount} entries as Stripe's do; whether Stripe's invoice-level total keeps the zero entry is unprobed.
3. Expanding discounts. expand[]=discounts silently does nothing (serialize/expand.py), so a discount's start/end can't be read through the API. Stripe's discount object has [id, object, checkout_session, customer, customer_account, end, invoice, invoice_item, promotion_code, source{coupon,type}, start, subscription, subscription_item] (probed).
4. Discount events. No customer.discount.created, customer.discount.updated or customer.discount.deleted events are emitted, including when a repeating discount ends. They are listed in spec/event_types.py but never fired.
5. Resume invoices. Resume invoices don't record their discounts or tax rates (no discounts= and no tax_rate_ids= on create_invoice). The totals list a di_ that invoice.discounts lacks, and a draft rebuild would drop both.

Impact on the company fixtures

• Item 1 would change the metadata on every plan line in both fixtures.
• Item 4 would add events.
• The other items have no fixture impact known.

Suggested next steps

Fix item 1 and item 5 first. Item 1 is visible on every invoice, and item 5 can drop a discount on rebuild. Then decide whether expand and discount events belong in scope.