Declarative hooks: add receipt QR and footer variable registry

[sridharkalaibala]

Parent: #34

Goal

Prepare safe receipt customisation without allowing arbitrary code inside billing.

Scope

Define the allowed variables an app can request for receipt extra lines, QR labels, and footer text.

Acceptance criteria

• Registry lists variable names, examples, source entity, and privacy risk.
• It excludes secrets, payment credentials, private customer notes, and raw health/tax documents.
• A sample config shows one QR line and one footer line.
• Fallback behavior is defined when a variable is missing.

Out of scope

• No production credentials, customer records, tax IDs, payment data, or private payloads in the repo or issue comments.
• Do not claim a live integration is supported until a maintainer accepts the matching fixture and evidence.