feat(billing): charge UK VAT on subscriptions

[mattwoberts]

Summary

Following Fider's switch from Paddle to Stripe (where we became the merchant of record), this adds UK VAT collection for UK-domiciled customers. Uses Stripe Tax Rates (the free legacy feature) — not the paid Stripe Tax — keeping ongoing cost at zero.

• Stripe Checkout is moved to mode: setup so the customer's billing address is captured before the first invoice is generated.
• The webhook then creates the Subscription server-side via the Stripe API and attaches a UK VAT tax rate (20%, inclusive) when the address country is GB. VAT appears on invoice secure the API #1, not just from invoice configure branches and its deployment pipeline #2.
• New env var STRIPE_UK_VAT_TAX_RATE_ID points at a Tax Rate object that needs to be created in the Stripe dashboard (live + test).

Notes

• All UK customers get 20% VAT — UK→UK is never reverse-charged, so no VAT-ID collection is needed.
• The legacy mode: subscription webhook path is retained to drain any in-flight Checkout sessions created before the deploy.
• Subscription creation uses session.ID as the Stripe idempotency key, so webhook retries are safe and won't create duplicates.
• The UX shifts slightly: Stripe Checkout in setup mode shows "Add a payment method" rather than "Subscribe to Pro — $25/month". A custom_text.submit.message is set so the customer still sees what they're committing to inside Checkout.

Test plan

• Test mode: create the Tax Rate, set env var, run through Checkout with UK test card 4000 0082 6000 0000 and confirm first invoice has VAT line.
• Test mode: run through Checkout with non-UK test card 4242 4242 4242 4242 and confirm no VAT applied.
• Verify webhook retries are idempotent (replay a checkout.session.completed event via Stripe CLI; only one subscription created).
• Verify cancellation still flips the tenant back to free.

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