feat(relay): implement POST /v1/accounts/mobile (MM-84)
Combined mobile account creation endpoint for the iOS identity wallet
onboarding flow. Atomically redeems a claim code, creates a pending
account, registers the device, and issues a pending session token in a
single transaction — with full rollback on any step failure.
- V007 migration: pending_sessions table (token_hash UNIQUE, FKs to
pending_accounts and devices) for pre-DID session tokens
- ClaimCodeRedeemed ErrorCode (409) to distinguish already-redeemed
codes from invalid/expired ones (404) per spec
- validate_handle and is_valid_platform promoted to pub(crate) for reuse
- Bruno collection entry for the new route
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