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,
  deadnix); 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)
  + disable empty_pattern via statix.toml (nixpkgs standard)
- Clean up unused lambda patterns across 38 .nix files via deadnix
- Format whole repo with alejandra (27 files)
- Remove .github/workflows/ci.yml (Gitea shadows .github; runner labels differ)
- Fix CI nix-not-found: export /run/current-system/sw/bin in PATH
- Trim aarch64 from pre-commit matrix (no QEMU binfmt deployed yet)

servers/hetzner/modules/services/caddy.nix

@@ -1,10 +1,4 @@
-{
-  config,
-  pkgs,
-  lib,
-  ...
-}: let
-  domain = "severijnse.eu";
+{...}: let
   antiScrape = ''
     @bad_bot {
       header_regexp User-Agent "(?i)(scrapy|cpython-requests|python-requests|curl|wget|go-http-client|ltx71|petalbot|bytespider|dotbot|ahrefsbot|semrushbot|mj12bot|dataforseo|facebookexternalhit|claudebot|anthropic-ai|perplexity|gptbot|chatgpt-user|omnisci|imgproxy|ccbot|exabot|360spider|baiduspider|sogou|duckduckgo|amazonbot|cohere-ai|diffbot|imagesiftbot).*"