feat(security): implement metadata redaction and sops-nix migration

Migrated authorized SSH keys and personal metadata (emails, tokens) to sops-nix to prevent infrastructure fingerprinting. Introduced centralized secrets module with placeholder fallbacks.

modules/apps/gitea/default.nix

@@ -496,7 +496,8 @@ in {
           #FORGEJO__mailer__CLIENT_KEY_FILE = "custom/mailer/key.pem";
           # Mail from address, RFC 5322. This can be just an email address, or the
           # `"Name" <email@example.com>` format.
-          FORGEJO__mailer__FROM = ''"${title}" <git@depeuter.dev>'';
+          # TODO Hugo: Populate 'gitea_mailer_from' in sops.
+          FORGEJO__mailer__FROM = config.sops.placeholder.gitea_mailer_from or "git@example.com";
           # Sometimes it is helpful to use a different address on the envelope. Set this to use
           # ENVELOPE_FROM as the from on the envelope. Set to `<>` to send an empty address.
           #FORGEJO__mailer__ENVELOPE_FROM = "";