feat: cancellation feedback funnel with existing Telegram alerts

[mutonby]

Summary

• Optional reason, private comment and rating before Stripe cancellation; equally visible skip.
• Authenticated server-side ownership, durable feedback intent records and account-erasure support.
• Forward submitted feedback via existing Telegram config after responding, with bounded plain text and heuristic PII redaction.
• Form discloses Telegram sharing; Stripe still confirms cancellation.

Verification

• 71 targeted Python tests passed (one existing Starlette/httpx deprecation warning).
• Dashboard build and component ESLint passed.
• Browser exercised opening, Escape/focus restoration, submit and skip with intercepted mock API, no live cancellation.
• Independent read-only review found no clear technical blockers.

Rollout / limitations

• Not deployed. PostgreSQL bootstrap creates the new table; production DB and actual Stripe test-mode portal handoff not exercised.
• Telegram delivery is best-effort: no durable retry queue or deduplication. No live Telegram test sent.
• DB persistence is awaited; a slow DB can delay portal response until frontend fallback.
• Redaction is heuristic, not anonymization; account erasure does not retract Telegram messages. Admin-chat retention/access must be managed.
• Build warns about missing local OpenPanel env, stale Browserslist and large chunks.
  See docs/cancellation-feedback.md.