feat: close the Stripe-compat gaps the platform billing flows hit

[midagedev]

What

A caller census against a Stripe SDK 31.1.0 client (plan upgrade, cancellation,
seat changes, member invites, dunning, one-time export purchases) turned up
parameters this stub rejected or silently ignored. 17 gaps, each with a file:line
citation on both sides.

Rejected → accepted and honoured

• invoiceitems pricing[price]/quantity (amount derived), items created without
  an invoice (pending, swept into the next include-behaviour invoice)
• invoices subscription; credit notes out_of_band_amount/memo
• customers invoice_settings[default_payment_method]
• subscriptions create proration_behavior, update standalone cancel_at,
  delete cancellation_details
• prices transfer_lookup_key

Silently ignored → honoured

• subscription period follows the price's recurring interval instead of a hardcoded
  month, so a yearly plan no longer ends after one month
• subscription list current_period_end range filters
• trial_end=now ends the trial instead of only recording evidence
• billing_cycle_anchor=now resets the period under every proration behaviour

New endpoints: invoices void / mark_uncollectible, checkout sessions
expire — each with its webhook event.

Response shapes: discount.source.coupon object (SDK 31 dropped the top-level
coupon), cancel_at serialized independently of cancel_at_period_end.

Migration

020 makes invoice_items.invoice_id nullable (SQLite table rebuild; columns carried
over verbatim from 015) and adds the credit-note out-of-band columns.

Verification

gofmt -l empty, go vet ./... clean, go test ./... -count=1 green across all 12
packages from cold. 16 new regression tests, one per gap — no existing assertions were
relaxed.

Not included (with reasons)

• Invoice lines.data materialization — totals and status are already correct; only
  line-level display is affected, and it needs the renewal line model.
• expand of invoice_settings.default_payment_method as an object — callers fall
  back to payment_methods.list.
• Future-dated billing_cycle_anchor scheduling — needs a scheduler model.