Implement backend support for recurring subscriptions

[yuvraj-shishodia]

Description

Implement backend support for recurring subscriptions while keeping the existing
one-time donation flow fully intact. This includes new authenticated endpoints
for setup intents and subscription creation, per-campaign Stripe product and
price management, and webhook updates to persist subscription and invoice data
in Firestore.

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Scope

Backend Endpoints

• Add createSetupIntent.ts HTTPS function to:
  • Verify Firebase ID token
  • Ensure or reuse a Stripe customer
  • Return SetupIntent clientSecret and customerId for saving payment methods
• Add createSubscription.ts HTTPS function to:
  • Verify authentication
  • Get or create a recurring Stripe price per campaign and interval
  • Attach payment method to customer
  • Create Stripe subscription
  • Return subscription status or paymentIntentClientSecret when 3DS is required
• Export new functions from index.ts
• Add shared request/response types in subscription.ts (+ index export)

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Subscription & Pricing Logic

Campaign Products

• Use a per-campaign Stripe product
• Store product ID on {campaignId}.billingProductId
• Create the product if missing, then reuse for all recurring prices

Pricing

• Support recurring intervals:
  • Monthly (1 month)
  • Quarterly (3 months)
  • Yearly (1 year)
• Currency defaults to EUR
• Custom donation amounts supported:
  • Server accepts amount in minor units
  • Matching recurring price is created or reused on the campaign product

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Webhook Integration

• Extend handleStripeWebhook.ts to:
  • Upsert subscriptions/{subscriptionId} documents
  • Upsert donations/{invoiceId} documents for recurring invoices
  • Increment campaign totals once per successful invoice (idempotent)
• Preserve existing one-time donation handling for
  payment_intent.succeeded without changes

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Acceptance Criteria

• SetupIntent endpoint implemented and authenticated
• Subscription creation endpoint implemented and authenticated
• Per-campaign Stripe product created and reused correctly
• Dynamic recurring prices supported per amount and interval
• Monthly, quarterly, and yearly subscriptions supported
• Webhook persists subscriptions and invoice-backed donations
• Campaign totals incremented idempotently
• One-time donation flow remains unchanged
• No regressions introduced

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Notes

• Frontend wiring for recurring donations will be tracked separately
• Relies on existing Stripe webhook infrastructure and Firestore schema