v4: pair relay devices with one-time QR tokens

[Phloraxx]

Scope

Eighth stacked PayGate v4 implementation slice. Depends on PR #49.

Implements

• authenticated server-side creation primitive for short-lived one-time pairing sessions
• cryptographically random 256-bit URL-safe pairing token; only SHA-256 token hash is stored
• default 2-minute TTL and single-use atomic consumption
• explicit replaceExisting intent for the one-active-relay-device model
• refuses silent replacement of an already-active phone
• replacement atomically disables the previous device before enabling the new one
• derives device ID from SHA-256 of P-256 SubjectPublicKeyInfo DER
• reuses existing P-256 ECDSA request-signing model; server stores public key only
• preserves device metadata and exposes active-device/revoke primitives
• older unconsumed pairing sessions are invalidated when a new pairing session is created
• no admin password, permanent relay secret, or matching state is encoded in the pairing token

Tests

Focused tests cover one-time use, expiry, invalid token, active-device refusal, explicit replacement, same-key re-enrollment, P-256 validation, token entropy/length, active-device lookup and revoke behavior. Full internal/v4/... unit tests, race tests, vet and diff checks pass locally.

Safety

Still no production HTTP route registration or Android rollout. This is an isolated service layer for the future QR/App-Link pairing UX.