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>

docker-compose.yml

@@ -53,6 +53,7 @@ services:
         VITE_AUTH0_CLIENT_ID: ${VITE_AUTH0_CLIENT_ID:-yspR8zdnSxmV8wFIghHynQ08iXAPoQJ3}
         VITE_AUTH0_AUDIENCE: ${VITE_AUTH0_AUDIENCE:-https://api.motovaultpro.com}
         VITE_API_BASE_URL: ${VITE_API_BASE_URL:-/api}
+        VITE_STRIPE_PUBLISHABLE_KEY: ${VITE_STRIPE_PUBLISHABLE_KEY:-}
     container_name: mvp-frontend
     restart: unless-stopped
     environment:
@@ -121,8 +122,8 @@ services:
       - ./secrets/app/resend-api-key.txt:/run/secrets/resend-api-key:ro
       - ./secrets/app/auth0-management-client-id.txt:/run/secrets/auth0-management-client-id:ro
       - ./secrets/app/auth0-management-client-secret.txt:/run/secrets/auth0-management-client-secret:ro
-      - ./secrets/app/stripe-secret-key.txt:/run/secrets/stripe-secret-key
-      - ./secrets/app/stripe-webhook-secret.txt:/run/secrets/stripe-webhook-secret
+      - ./secrets/app/stripe-secret-key.txt:/run/secrets/stripe-secret-key:ro
+      - ./secrets/app/stripe-webhook-secret.txt:/run/secrets/stripe-webhook-secret:ro
       # Filesystem storage for documents
       - ./data/documents:/app/data/documents
       # Filesystem storage for backups