fix: disable systemd-oomd memory pressure kill for runner users

The default systemd user service configuration (10-oomd-user-service-defaults.conf)
sets ManagedOOMMemoryPressure=kill with ManagedOOMMemoryPressureLimit=50%.
Under parallel DinD load (6 molecule tests), memory pressure exceeds 50%
for 30 seconds, causing systemd-oomd to SIGKILL molecule containers
(rc=137). This was the root cause of recurring "container is not running"
and "Failed to create temporary directory" errors in CI.

Fix: Create a per-user systemd override that sets:
- ManagedOOMMemoryPressure=auto (don't kill on pressure)
- ManagedOOMMemoryPressureLimit=100% (never trigger)
- OOMScoreAdjust=-500 (very unlikely to be OOM-killed)

The runner users run Docker-in-Docker workloads that legitimately use
large amounts of memory. systemd-oomd's aggressive memory pressure
killing is inappropriate for CI runner workloads.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

ansible/roles/gitea_runner/handlers/main.yml

@@ -10,3 +10,7 @@
     - ansible_facts is defined
     - ansible_facts['service_mgr'] | default('') == 'systemd'
     - gitea_runner_docker_rootless_setup
+
+- name: Reload systemd user daemon
+  ansible.builtin.systemd:
+    daemon_reload: true

ansible/roles/gitea_runner/tasks/user_setup.yml

@@ -69,3 +69,31 @@
     owner: "{{ gitea_runner_service_user }}"
     group: "{{ gitea_runner_service_user }}"
     mode: "0755"
+
+- name: Disable systemd-oomd memory pressure kill for runner user
+  when: gitea_runner_systemd_available.stat.exists
+  block:
+    - name: Ensure user service override directory exists
+      ansible.builtin.file:
+        path: "/etc/systemd/system/user@{{ gitea_runner_uid }}.service.d"
+        state: directory
+        owner: root
+        group: root
+        mode: "0755"
+
+    - name: Disable ManagedOOMMemoryPressure for runner user
+      ansible.builtin.copy:
+        content: |
+          [Service]
+          ManagedOOMMemoryPressure=auto
+          ManagedOOMMemoryPressureLimit=100%
+          OOMScoreAdjust=-500
+        dest: "/etc/systemd/system/user@{{ gitea_runner_uid }}.service.d/oomd-override.conf"
+        owner: root
+        group: root
+        mode: "0644"
+      notify: Reload systemd user daemon
+
+    - name: Reload systemd daemon for oomd override
+      ansible.builtin.systemd:
+        daemon_reload: true