feat: implement x402 seller workflows

[nkavian]

Summary

• Add async Seller configuration, exact prices, route construction, sponsorship declarations, and upstream scheme registrations.
• Add an InFlow Facilitator with stable payment identifiers, precise rejection handling, and bounded retries for explicit pending settlements only.
• Document FastAPI setup, metering, client lifetime, and upstream integration behavior.

Verification

• make sync and make verify passed on Python 3.11, 3.12, 3.13, and 3.14: formatting, strict typing, 757 tests, coverage gates, package build, and isolated consumers.
• All 39 shared seller vectors match inflow-specs.
• Real upstream FastAPI middleware tests cover successful and rejected payments, handler failures, and metered settlement. InFlow HTTP responses are mocked; no live funds were moved.
• Review covered cancellation, cache isolation, payment identifier stability, optional imports, retry safety, and independent sponsorship schemas.

Related changes

• Fixture correction: fix: include networks in settlement failure fixtures inflow-specs#17
• Server settlement response correction is merged in https://github.com/inflowpayai/inflow-server/pull/705; deployment has not been verified.

Broader cross-product hardening, runnable example applications, and cross-language interoperability remain separate queued tasks.