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>

frontend/Dockerfile

@@ -19,15 +19,17 @@ FROM deps AS build
 
 # Accept build arguments for environment variables
 ARG VITE_AUTH0_DOMAIN
-ARG VITE_AUTH0_CLIENT_ID  
+ARG VITE_AUTH0_CLIENT_ID
 ARG VITE_AUTH0_AUDIENCE
 ARG VITE_API_BASE_URL
+ARG VITE_STRIPE_PUBLISHABLE_KEY
 
 # Set environment variables from build args
 ENV VITE_AUTH0_DOMAIN=$VITE_AUTH0_DOMAIN
 ENV VITE_AUTH0_CLIENT_ID=$VITE_AUTH0_CLIENT_ID
 ENV VITE_AUTH0_AUDIENCE=$VITE_AUTH0_AUDIENCE
 ENV VITE_API_BASE_URL=$VITE_API_BASE_URL
+ENV VITE_STRIPE_PUBLISHABLE_KEY=$VITE_STRIPE_PUBLISHABLE_KEY
 
 COPY . .
 RUN npm run build