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

flake.lock

@@ -82,6 +82,22 @@
         "type": "github"
       }
     },
+    "flake-compat_2": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1767039857,
+        "narHash": "sha256-vNpUSpF5Nuw8xvDLj2KCwwksIbjua2LZCqhV1LNRDns=",
+        "owner": "NixOS",
+        "repo": "flake-compat",
+        "rev": "5edf11c44bc78a0d334f6334cdaf7d60d732daab",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "repo": "flake-compat",
+        "type": "github"
+      }
+    },
     "flake-parts": {
       "inputs": {
         "nixpkgs-lib": [
@@ -122,6 +138,27 @@
         "type": "github"
       }
     },
+    "git-hooks-nix": {
+      "inputs": {
+        "flake-compat": "flake-compat_2",
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1783008725,
+        "narHash": "sha256-jGiy6+sxjNWXSjp25uoJuNfyH9zBK1PEDY0lVoL4ibQ=",
+        "owner": "cachix",
+        "repo": "git-hooks.nix",
+        "rev": "bca82caa46d5ec0f5d422c61fb1e30bc51313cbe",
+        "type": "github"
+      },
+      "original": {
+        "owner": "cachix",
+        "repo": "git-hooks.nix",
+        "type": "github"
+      }
+    },
     "helium": {
       "inputs": {
         "nixpkgs": [
@@ -254,6 +291,7 @@
         "flake-compat": "flake-compat",
         "flake-parts": "flake-parts",
         "flake-utils": "flake-utils",
+        "git-hooks-nix": "git-hooks-nix",
         "helium": "helium",
         "import-tree": "import-tree",
         "nix-index-db": "nix-index-db",