chore(db): backfill purchase_history.cloud_account_id from (provider, external_id) #22

Follow-up to the account-filter representation fix

The account-filter fix (rows 305/306/384/431/452, issues LeanerCloud/cloud-commitments-cli#701/#498/#866) resolves filtering by matching BOTH purchase_history.cloud_account_id (UUID) and account_id (provider external id) at query time, because cloud_account_id is NULL on many rows (migration 000011 added it without a backfill; direct-execute / ambient / legacy write paths leave it NULL).

This issue tracks the optional one-time backfill to converge historical rows onto the UUID column so future code can rely on it:

UPDATE purchase_history ph
SET cloud_account_id = ca.id
FROM cloud_accounts ca
WHERE ph.account_id = ca.external_id
  AND ph.provider   = ca.provider
  AND ph.cloud_account_id IS NULL;

Notes / risks

• Must match on (provider, external_id) (the cloud_accounts UNIQUE(provider, external_id) constraint), NOT external_id alone, to avoid cross-provider/tenant collisions.
• Not required for correctness (the resolve-at-query fix handles it); this is cleanup/convergence only.
• After backfill + a soak period, the dual-column predicate could be simplified back to UUID-only (separate change).

Acceptance criteria

• Migration backfills cloud_account_id for all rows with a resolvable (provider, external_id).
• Rows with no matching cloud_accounts entry are left NULL (and still filterable via the external-id branch).
• A test verifies the backfill maps correctly and does not mis-assign across providers.