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

.github/workflows/ci.yml

@@ -45,8 +45,8 @@ jobs:
       - name: Flake check (${{ matrix.system }})
         run: nix flake check --no-build --system ${{ matrix.system }}
 
-  format:
-    name: Formatting check (${{ matrix.system }})
+  pre-commit:
+    name: Pre-commit checks (${{ matrix.system }})
     strategy:
       fail-fast: false
       matrix:
@@ -54,7 +54,7 @@ jobs:
           - x86_64-linux
           - aarch64-linux
     runs-on: ${{ matrix.system == 'aarch64-linux' && 'ubuntu-24.04-arm' || 'ubuntu-latest' }}
-    timeout-minutes: 10
+    timeout-minutes: 20
     steps:
       - uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
         with:
@@ -65,7 +65,10 @@ jobs:
           extra_nix_config: |
             experimental-features = nix-command flakes
 
-      # alejandra is the configured formatter (flake.nix perSystem.formatter).
-      # Checked per-system so both architectures stay formatted.
-      - name: Formatting check (${{ matrix.system }})
-        run: nix run .#formatter.${{ matrix.system }} -- --check .
+      # Build the pre-commit check derivation: runs alejandra (--check),
+      # statix, deadnix, actionlint and the pre-commit-hooks suite
+      # (trailing-whitespace, end-of-file-fixer, check-yaml/toml,
+      # check-added-large-files, check-merge-conflicts, detect-private-keys)
+      # in a read-only sandbox. Fails the build on any reported problem.
+      - name: Pre-commit checks (${{ matrix.system }})
+        run: nix build .#checks.${{ matrix.system }}.pre-commit