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>

playbooks/runner.yml

@@ -0,0 +1,131 @@
+- name: Set up runner host via jumphost.
+  gather_facts: false
+  hosts: localhost
+  vars_files:
+    - ../vault.yml
+    - ../dist/terraform_outputs.yml
+  tasks:
+    - name: Add gitea server as jumphost.
+      ansible.builtin.add_host:
+        name: server
+        ansible_ssh_host: "{{ server_ip.value }}"
+        ansible_user: root
+        ansible_port: 2222
+        ansible_private_key_file: "{{ secret.private_ssh_key_path }}"
+
+    - name: Add runner host (via jumphost).
+      ansible.builtin.add_host:
+        name: runner
+        ansible_ssh_host: "{{ runner_ip.value }}"
+        ansible_user: root
+        ansible_private_key_file: "{{ secret.private_ssh_key_path }}"
+        ansible_ssh_common_args: >-
+          -o ProxyCommand="ssh -i {{ secret.private_ssh_key_path }} -p 2222 -W %h:%p root@{{ server_ip.value }}"
+
+- name: Install Docker on runner.
+  gather_facts: true
+  hosts: runner
+  vars_files:
+    - ../vault.yml
+    - ../dist/terraform_outputs.yml
+  tasks:
+    - name: Set DNS resolver.
+      ansible.builtin.copy:
+        content: "nameserver 185.12.64.2\n"
+        dest: /etc/resolv.conf
+        mode: "0644"
+
+    - name: Install PIP.
+      ansible.builtin.apt:
+        state: present
+        update_cache: true
+        name:
+          - python3-pip
+
+    - name: Install needed packages.
+      ansible.builtin.pip:
+        name:
+          - packaging
+        state: present
+        break_system_packages: true
+
+    - name: Download Docker repository key.
+      ansible.builtin.apt_key:
+        url: https://download.docker.com/linux/debian/gpg
+        state: present
+
+    - name: Download Docker repository.
+      ansible.builtin.apt_repository:
+        repo: "deb https://download.docker.com/linux/debian {{ ansible_distribution_release }} stable"
+        state: present
+
+    - name: Remove bad packages.
+      ansible.builtin.apt:
+        state: absent
+        package:
+          - docker.io
+          - docker-doc
+          - docker-compose
+          - podman-docker
+          - containerd
+          - runc
+
+    - name: Download Docker dependencies.
+      ansible.builtin.apt:
+        state: present
+        package:
+          - ca-certificates
+          - curl
+          - jq
+
+    - name: Download Docker packages.
+      ansible.builtin.apt:
+        state: present
+        update_cache: true
+        package:
+          - docker-ce
+          - docker-ce-cli
+          - containerd.io
+          - docker-buildx-plugin
+          - docker-compose-plugin
+
+- name: Register and start Gitea runner.
+  hosts: runner
+  gather_facts: false
+  vars_files:
+    - ../vault.yml
+    - ../dist/terraform_outputs.yml
+  vars:
+    gitea_internal_url: "https://{{ server_fqdn.value }}"
+  tasks:
+    - name: Create runner data volume.
+      community.docker.docker_volume:
+        name: runner-data
+        state: present
+
+    - name: Generate runner config.
+      ansible.builtin.copy:
+        dest: /root/runner-config.yaml
+        mode: "0644"
+        content: |
+          runner:
+            insecure: true
+
+    - name: Start Gitea runner container.
+      community.docker.docker_container:
+        name: gitea-runner
+        image: gitea/act_runner:latest
+        state: started
+        recreate: true
+        restart_policy: unless-stopped
+        etc_hosts:
+          "{{ server_fqdn.value }}": "10.0.1.2"
+        volumes:
+          - runner-data:/data
+          - /var/run/docker.sock:/var/run/docker.sock
+          - /root/runner-config.yaml:/config.yaml:ro
+        env:
+          GITEA_INSTANCE_URL: "{{ gitea_internal_url }}"
+          GITEA_RUNNER_REGISTRATION_TOKEN: "{{ secret.runner_registration_token }}"
+          GITEA_RUNNER_NAME: "runner-01"
+          CONFIG_FILE: "/config.yaml"