🔒 GDPR billing org-axis data provider (anonymize, keep invoices)

[PierreBrisorgueil]

Self-register billing as an org-axis GDPR provider that anonymizes payment identifiers while keeping the financial record (legal/tax retention).

Scope — billing.init.js (axis:'org', retention:'anonymize')

• Export per orgId: Subscription + ExtraBalance ledger + Usage (exclude internal ProcessedStripeEvent / FailedBackfill — not subject data).
• Erase = anonymize Stripe identifiers (stripeCustomerId, stripeSubscriptionId, ledger stripeSessionId / historyId / adminUpdatedBy); keep org id, amounts, dates, ledger kinds (tax retention). BillingUsage (pure metering) may hard-delete (add deleteByOrg).
• Degrades to a no-op when billing is deactivated.

DoD

• Provider resolves orgs via the user→org bridge; anonymize keeps the financial trail intact.
• /verify green.

Note: a billing org-removal cleanup handler is a separate concern, out of scope here.

Depends on: GDPR export+erasure controller (org-axis payload).

Created via /dev:issue