docs: add homelab secrets setup instructions

HOMELAB_SECRETS_SETUP.md

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+# Homelab Deployment Secrets Setup
+
+The release workflow now includes automatic deployment to the homelab runner server. You need to add these secrets to Gitea.
+
+## Required Secrets
+
+Go to: `https://gitea.kalekber.cc/kamil/libnovel/settings/secrets/actions`
+
+### 1. HOMELAB_HOST
+```
+192.168.0.109
+```
+
+### 2. HOMELAB_USER
+```
+root
+```
+
+### 3. HOMELAB_SSH_KEY
+
+If you want to use the same SSH key as prod:
+- Copy the value from `PROD_SSH_KEY` secret
+
+If you want a separate key:
+```bash
+# On your local machine or CI runner
+cat ~/.ssh/id_rsa  # or your preferred key
+```
+
+### 4. HOMELAB_SSH_KNOWN_HOSTS
+
+Run this when the homelab server is reachable:
+```bash
+ssh-keyscan -H 192.168.0.109 2>/dev/null
+```
+
+Expected output format:
+```
+|1|base64hash...|192.168.0.109 ssh-rsa AAAAB3NzaC...
+|1|base64hash...|192.168.0.109 ecdsa-sha2-nistp256 AAAAE2...
+|1|base64hash...|192.168.0.109 ssh-ed25519 AAAAC3...
+```
+
+## Testing
+
+After adding the secrets, the next release (e.g., v4.1.10) will automatically:
+1. Build all Docker images
+2. Deploy to prod (165.22.70.138) ✅
+3. Deploy to homelab (192.168.0.109) ✅ NEW
+4. Create a Gitea release
+
+Both deployments run in parallel for faster releases.
+
+## Troubleshooting
+
+If the homelab deployment fails:
+- Check that the secrets are set correctly
+- Verify SSH access: `ssh root@192.168.0.109`
+- Check Doppler config exists: `doppler configs --project libnovel`
+- Manually test: `cd /opt/libnovel-runner && doppler run --project libnovel --config prd_homelab -- docker compose pull runner`