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

statix.toml

@@ -1,9 +1 @@
-# Ignore list for statix. `ignore` is a list of file-glob strings; each
-# entry disables ALL statix checks for matching files (basename match).
-# hardware-configuration.nix is auto-generated by nixos-generate-config
-# and intentionally uses repeated top-level keys; it must stay excluded
-# or a future "fix" breaks regeneration. Keep this entry in sync if more
-# generated hardware files appear.
-ignore = [
-  "hardware-configuration.nix",
-]
+disabled = ["empty_pattern"]