feat: add Gitea Actions runner (#6)

## Summary
- Adds a private runner server on the Hetzner private network (no public IP)
- NAT through the gitea server for outbound internet access via `hcloud_network_route` and iptables forwarding rules
- Runner connects to gitea over HTTPS on the private network with TLS verification disabled
- Includes Taskfile commands for runner deployment and SSH access

## Test plan
- [x] Runner registers with gitea instance
- [x] Private network connectivity verified
- [ ] Run a test workflow to confirm end-to-end CI

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

Reviewed-on: #6
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

gitea/config/app.ini

@@ -91,6 +91,9 @@ DEFAULT_MERGE_STYLE = merge
 [repository.signing]
 DEFAULT_TRUST_MODEL = committer
 
+[actions]
+ENABLED = true
+
 [storage]
 STORAGE_TYPE = minio
 MINIO_USE_SSL = true

gitea/config/dev.app.ini

@@ -96,6 +96,9 @@ DEFAULT_TRUST_MODEL = committer
 [oauth2]
 JWT_SECRET = x-----------------------------------------x
 
+[actions]
+ENABLED = true
+
 [storage]
 STORAGE_TYPE = minio
 MINIO_ENDPOINT = localstack:4566