gitea: add actions runner and CI/CD deploy workflow

- enable gitea actions
- add native host runner (nix:host label, capacity 1)
- add gitea-runner system user with persisted state
- add agenix-encrypted CI secrets (deploy key, git-crypt key, runner token)
- authorize CI deploy key for root SSH
- add build-and-deploy workflow triggered on push to main

.gitea/workflows/deploy.yml

@@ -0,0 +1,48 @@
+name: Build and Deploy
+on:
+  push:
+    branches: [main]
+
+jobs:
+  deploy:
+    runs-on: nix
+    steps:
+      - uses: https://github.com/actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Build NixOS configuration
+        run: |
+          nix build .#nixosConfigurations.muffin.config.system.build.toplevel -L
+
+      - name: Deploy via deploy-rs
+        run: |
+          eval $(ssh-agent -s)
+          ssh-add /run/agenix/ci-deploy-key
+          nix run github:serokell/deploy-rs -- .#muffin --ssh-opts="-o StrictHostKeyChecking=no"
+
+      - name: Health check
+        run: |
+          sleep 10
+          ssh -i /run/agenix/ci-deploy-key -o StrictHostKeyChecking=no root@server-public \
+            "systemctl is-active gitea && systemctl is-active caddy && systemctl is-active continuwuity && systemctl is-active coturn"
+
+      - name: Notify success
+        if: success()
+        run: |
+          curl -sf -X POST \
+            "https://ntfy.sigkill.computer/deployments" \
+            -H "Title: [muffin] Deploy succeeded" \
+            -H "Priority: default" \
+            -H "Tags: white_check_mark" \
+            -d "server-config deployed from commit ${GITHUB_SHA::8}"
+
+      - name: Notify failure
+        if: failure()
+        run: |
+          curl -sf -X POST \
+            "https://ntfy.sigkill.computer/deployments" \
+            -H "Title: [muffin] Deploy FAILED" \
+            -H "Priority: urgent" \
+            -H "Tags: rotating_light" \
+            -d "server-config deploy failed at commit ${GITHUB_SHA::8}"