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

@@ -1,9 +1,4 @@
-{
-  config,
-  pkgs,
-  lib,
-  ...
-}: let
+{pkgs, ...}: let
   # Caddy's canonical certificate storage (XDG data dir). Renewals land here,
   # owned caddy:caddy 0600 — the mail server's non-root Postfix/Dovecot cannot
   # read it directly, so we copy it into a world-readable distribution dir.