fix: add Stripe secrets to CI/CD and build configuration (refs #55)

- Add VITE_STRIPE_PUBLISHABLE_KEY to frontend Dockerfile build args
- Add VITE_STRIPE_PUBLISHABLE_KEY to docker-compose.yml build args
- Add :ro flag to backend Stripe secret volume mounts for consistency
- Update inject-secrets.sh with STRIPE_SECRET_KEY and STRIPE_WEBHOOK_SECRET
- Add Stripe secrets to staging.yaml workflow (build arg + inject step)
- Add Stripe secrets to production.yaml workflow (inject step)

Requires STRIPE_SECRET_KEY, STRIPE_WEBHOOK_SECRET secrets and
VITE_STRIPE_PUBLISHABLE_KEY variable to be configured in Gitea.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitea/workflows/production.yaml

@@ -119,6 +119,8 @@ jobs:
           GOOGLE_MAPS_MAP_ID: ${{ secrets.GOOGLE_MAPS_MAP_ID }}
           CF_DNS_API_TOKEN: ${{ secrets.CF_DNS_API_TOKEN }}
           RESEND_API_KEY: ${{ secrets.RESEND_API_KEY }}
+          STRIPE_SECRET_KEY: ${{ secrets.STRIPE_SECRET_KEY }}
+          STRIPE_WEBHOOK_SECRET: ${{ secrets.STRIPE_WEBHOOK_SECRET }}
 
       - name: Initialize data directories
         run: |