Fix legacy Stripe customer linking

[ComputelessComputer]

Match billing identities case-insensitively, prefer customers with subscription history, and backfill unlinked profiles.

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Note

High Risk
Changes SECURITY DEFINER auth triggers and a one-time profile backfill that sets stripe_customer_id, which directly affects billing identity and could mis-link or skip subscriptions if matching rules are wrong.

Overview
Replaces naive Stripe auto-linking on signup and email change with case-insensitive, trimmed email matching, transactional advisory locks per email, and deterministic customer choice when multiple Stripe records share an address.

Linking now skips deleted Stripe customers and IDs already tied to another profiles row or workspace billing. Among eligible matches it prefers customers with active/past_due/unpaid subscriptions, then trialing/paused, then any subscription history, then oldest created / stable id. Email updates only fill stripe_customer_id when it is still null, so existing links are not overwritten.

The first migration backfills null profiles.stripe_customer_id for legacy users (unique normalized email only, excluding pending account deletion jobs) using the same ranking rules. Follow-up migrations re-apply the hardened trigger definitions without changing that behavior. pgTAP coverage expands for mixed case, duplicate customers, deleted customers, workspace billing collisions, and email-update paths.

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