ops: staging environment with seeded testnet data

[joelpeace48-cell]

Problem

There is nowhere to try changes against realistic data before production, and
nowhere to point a pilot partner or a grant reviewer for a hands-on look. Demos
against a local machine are fragile and unconvincing.

What to do

• A staging deployment on Stellar testnet, mirroring production configuration.
• A seed script generating realistic data: multiple payers and models,
  attestation history, usage crossing quota boundaries, at least one open
  dispute and one resolved one.
• Reset-and-reseed on demand, so a broken demo is one command from fixed.
• Clearly badged as testnet in the UI, to remove any chance of confusion with
  real settlement.
• Deploy every merge to main automatically.

Acceptance criteria

• Staging deployed and auto-updating from main
• Seed script producing realistic, varied data
• One-command reset and reseed
• Unmissable testnet badging

Notes

Include a dispute in the seed data. It is the most compelling thing to
demonstrate and the most awkward to produce live.