fix: step 6 webhooks demo - replace 404 events API with webhook registration flow

- Replace /events/v2/* endpoints (404 in sandbox) with /api/v1/webhooks
- Add list_webhooks() and register_webhook() methods to TinkClient
- Step 6 now shows: webhook flow diagram + curl examples + live API + sample payload
- Handle sandbox 404 gracefully (shows example data, no red error)
- Remove .env.production from git tracking (credentials via Gitea secrets)
- deploy.yml: write .env.production from TINK_CLIENT_ID/SECRET secrets

Dockerfile

@@ -6,6 +6,7 @@ COPY requirements.txt .
 RUN pip install --no-cache-dir -r requirements.txt
 
 COPY src/ src/
+COPY .env.production .env
 
 EXPOSE 8000