Harden server and add Nix-native CI + self-hosted Gitea Actions

- caddy: security headers (X-Content-Type-Options/X-XSS-Protection/
  X-Frame-Options) on all vhosts + baseline CSP; strip SnappyMail
  upstream copies via header_down on mail.severijnse.eu
- tlsa-updater: compute TLSA 3 1 1 from cert SPKI (SHA-256), sync
  _25/_465/_993, fail-safe placeholders; coredns zone updated
- pre-commit: wire cachix/git-hooks.nix (alejandra, statix, actionlint,
  ...); CI pre-commit job over x86_64 + aarch64 matrix
- gitea: enable Gitea Actions + self-hosted runner (native:host,
  aarch64 via binfmt); add .gitea/workflows/ci.yml and local hook
- fix statix warnings (merge repeated systemd/database/configFile keys,
  inherit, bool-compare guards); add missing trailing newlines

servers/hetzner/modules/services/tlsa-updater.nix

@@ -58,42 +58,44 @@
     echo "tlsa-update: TLSA set to $HEX"
   '';
 in {
-  # Ensure the distribution dir exists (Caddy does not write here).
-  systemd.tmpfiles.rules = [
-    "d ${distCertDir} 0755 root root - -"
-  ];
+  systemd = {
+    # Ensure the distribution dir exists (Caddy does not write here).
+    tmpfiles.rules = [
+      "d ${distCertDir} 0755 root root - -"
+    ];
 
-  systemd.services.tlsa-update = {
-    description = "Sync Caddy TLS certificate to mail server and update DANE/TLSA records";
-    after = ["caddy.service" "coredns.service"];
-    partOf = ["coredns.service"];
-    wantedBy = ["multi-user.target"];
-    path = with pkgs; [openssl coreutils gnused podman systemd];
-    serviceConfig = {
-      Type = "oneshot";
-      ExecStart = "${syncScript}";
-      User = "root";
-      Group = "root";
+    services.tlsa-update = {
+      description = "Sync Caddy TLS certificate to mail server and update DANE/TLSA records";
+      after = ["caddy.service" "coredns.service"];
+      partOf = ["coredns.service"];
+      wantedBy = ["multi-user.target"];
+      path = with pkgs; [openssl coreutils gnused podman systemd];
+      serviceConfig = {
+        Type = "oneshot";
+        ExecStart = "${syncScript}";
+        User = "root";
+        Group = "root";
+      };
     };
-  };
 
-  # Fire as soon as Caddy rewrites the certificate on renewal (the atomic rewrite
-  # changes the directory mtime), eliminating the up-to-24h DANE drift window.
-  systemd.paths.tlsa-update = {
-    description = "Watch Caddy certificate directory for renewal";
-    wantedBy = ["paths.target"];
-    pathConfig = {
-      PathModified = [caddyCertDir];
-      Unit = "tlsa-update.service";
+    # Fire as soon as Caddy rewrites the certificate on renewal (the atomic rewrite
+    # changes the directory mtime), eliminating the up-to-24h DANE drift window.
+    paths.tlsa-update = {
+      description = "Watch Caddy certificate directory for renewal";
+      wantedBy = ["paths.target"];
+      pathConfig = {
+        PathModified = [caddyCertDir];
+        Unit = "tlsa-update.service";
+      };
     };
-  };
 
-  # Fallback in case a renewal event is missed (e.g. inotify overflow).
-  systemd.timers.tlsa-update = {
-    wantedBy = ["timers.target"];
-    timerConfig = {
-      OnCalendar = "daily";
-      Persistent = true;
+    # Fallback in case a renewal event is missed (e.g. inotify overflow).
+    timers.tlsa-update = {
+      wantedBy = ["timers.target"];
+      timerConfig = {
+        OnCalendar = "daily";
+        Persistent = true;
+      };
     };
   };
 }