feat: add Gitea self-hosted Git service with Actions runner

- Create new 'dev' stack at dev.pivoine.art
- Add Gitea with PostgreSQL database (core stack)
- Add Gitea Actions runner with Docker support
- Enable mailer configuration via SMTP
- Enable container registry and packages
- Add gitea database to PostgreSQL init script

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

Co-Authored-By: Claude <noreply@anthropic.com>

arty.yml

@@ -155,6 +155,14 @@ envs:
     MEDIA_FILESTASH_IMAGE: machines/filestash:latest
     MEDIA_FILESTASH_TRAEFIK_HOST: filestash.media.pivoine.art
     MEDIA_FILESTASH_CANARY: true
+    # Dev (Gitea + Runner)
+    DEV_TRAEFIK_ENABLED: true
+    DEV_COMPOSE_PROJECT_NAME: dev
+    DEV_GITEA_IMAGE: gitea/gitea:latest
+    DEV_GITEA_TRAEFIK_HOST: dev.pivoine.art
+    DEV_GITEA_DB_NAME: gitea
+    DEV_GITEA_RUNNER_IMAGE: gitea/act_runner:latest
+    DEV_GITEA_RUNNER_NAME: docker-runner
     # PairDrop
     DROP_TRAEFIK_ENABLED: true
     DROP_COMPOSE_PROJECT_NAME: drop