Fix #1466 surface subscription cancellation reason in admin UI

[chrisjsimpson]

Summary

Closes #1466.

Shop owners could previously see that a subscription had been cancelled, but the why (Stripe's cancellation_details.reason) was only visible on the shop-wide "recent cancellations" report, not on the subscribers list where they naturally look.

• Add Subscription.stripe_cancellation_reason column (+ alembic migration) populated from:
  • the customer.subscription.deleted Stripe webhook, and
  • the manual Refresh Status / background refresh paths (so historic cancellations can be backfilled).
• Render the reason next to the existing Date ended at line on both the subscribers list (/admin/subscribers) and the per-subscriber detail page, with human-friendly labels + explainer popovers for the three reasons Stripe emits (payment_failed, cancellation_requested, payment_disputed).
• Add Python regression test (tests/test_webhook_subscription_cancellation_reason.py) verifying the webhook now persists the reason, and a Playwright spec (1466_subscription_cancellation_reason_visible.spec.js) that records a video and captures screenshots of the reason in the UI.

Proof

(see attached 1466-subscribers-page.png, 1466-subscriber-detail-page.png, and issue-1466-cancellation-reason.webm for the Playwright video.)

Test plan

• pytest tests/ --ignore=tests/browser-automated-tests-playwright — 28 passed, 1 skipped
• flask db upgrade succeeds on existing SQLite DB (subscription.stripe_cancellation_reason column added)
• Playwright spec @1466_subscription_cancellation_reason_visible records video + screenshots and asserts the reason is visible on the subscribers list and detail pages
• Manual: with a real Stripe test-mode subscription cancelled via Cancel action, refresh the subscribers page and confirm the reason renders as "Cancellation requested"
• Manual: trigger a customer.subscription.deleted webhook with cancellation_details.reason = payment_failed and confirm it renders as "Payment failed"

🤖 Generated with Claude Code

[chrisjsimpson]

Playwright verification artifacts

Ran the new spec e2e/1466_subscription_cancellation_reason_visible.spec.js against a dev shop with two canceled subscriptions seeded (one payment_failed, one cancellation_requested):

• Screenshot — subscribers list: shows both cancellation reasons rendered with explainer popovers.
• Screenshot — subscriber detail: same reasons rendered on the per-subscriber page.
• Video (video.webm): records the login → subscribers list → subscriber detail flow, captured automatically by Playwright (video: 'on').

Local artifacts saved to /tmp/issue-1466-artifacts/ on the dev machine:

1466-subscribers-page.png          319 KB
1466-subscriber-detail-page.png    326 KB
issue-1466-cancellation-reason.webm 346 KB

Spec output:

[#1466] Reasons on subscribers page: [ 'Payment failed...', 'Cancellation requested...' ]
1 passed (17.5s)