feat(gateway): add security headers to all vhosts

X-Frame-Options, X-Content-Type-Options, Referrer-Policy at server
scope on _, forge, and matrix vhosts. HSTS added when TLS is active.

nginx inheritance rule: locations with their own add_header (CORS API
endpoints like /.well-known/matrix/client, /_matrix/) are unaffected —
they already carry the headers they need. HTML-serving and proxy
locations pick the security headers up automatically.

nix/modules/hive-gateway.nix

@@ -444,15 +444,9 @@ in
           # - selfSignedTls=true: generated cert stored in persistent state dir.
           # - tls.certDir set: operator-provided cert bind-mounted at /run/hive-tls.
           tlsCert =
-            if cfg.tls.certDir != null then
+            if cfg.tls.certDir != null then "/run/hive-tls/${cfg.tls.certName}" else "${tlsDir}/cert.pem";
-              "/run/hive-tls/${cfg.tls.certName}"
-            else
-              "${tlsDir}/cert.pem";
           tlsKey =
-            if cfg.tls.certDir != null then
+            if cfg.tls.certDir != null then "/run/hive-tls/${cfg.tls.keyName}" else "${tlsDir}/key.pem";
-              "/run/hive-tls/${cfg.tls.keyName}"
-            else
-              "${tlsDir}/key.pem";
           # True when nginx should listen with TLS (any mode).
           hasTls = cfg.selfSignedTls || cfg.tls.certDir != null || cfg.tls.acme.enable;
           # Listen addresses every vhost shares. Plain http on `cfg.port`
@@ -498,6 +492,19 @@ in
           publicPort = if hasTls then cfg.httpsPort else cfg.port;
           publicPortDefault = if hasTls then 443 else 80;
           publicPortSuffix = if publicPort == publicPortDefault then "" else ":${toString publicPort}";
+
+          # Security headers added at the server scope on every vhost.
+          # nginx's add_header inheritance rule: a location that defines its
+          # own add_header does NOT inherit the server-level ones, so API
+          # locations with CORS headers (e.g. /.well-known/matrix/client,
+          # /_matrix/) are unaffected. HTML-serving and proxy locations that
+          # carry no add_header of their own pick these up automatically.
+          securityHeaders = ''
+            add_header X-Frame-Options "SAMEORIGIN" always;
+            add_header X-Content-Type-Options "nosniff" always;
+            add_header Referrer-Policy "strict-origin-when-cross-origin" always;
+            ${lib.optionalString hasTls ''add_header Strict-Transport-Security "max-age=31536000; includeSubDomains" always;''}
+          '';
         in
         {
           system.stateVersion = "26.05";
@@ -794,7 +801,7 @@ in
                 # agents without a nixos-rebuild. nginx's longest-prefix-
                 # match rule ensures `/agent/<name>/` from this file beats
                 # the `/agent/` catch-all above.
-                extraConfig = ''
+                extraConfig = securityHeaders + ''
                   include /run/hive-state/agents.conf;
                 '';
               };
@@ -808,6 +815,7 @@ in
               lib.optionalAttrs (forgeCfg.enable or false && forgeCfg.behindGateway or false) {
                 "${forgeCfg.domain}" = vhostTls // {
                   listen = vhostListen;
+                  extraConfig = securityHeaders;
                   locations."/" = {
                     proxyPass = "http://127.0.0.1:${toString forgeCfg.httpPort}/";
                     proxyWebsockets = true;
@@ -830,6 +838,7 @@ in
               lib.optionalAttrs (matrixCfg.enable && matrixCfg.gatewayHost != null) {
                 "${matrixCfg.gatewayHost}" = vhostTls // {
                   listen = vhostListen;
+                  extraConfig = securityHeaders;
                   locations = {
                     "/_matrix/" = {
                       proxyPass = "http://127.0.0.1:${toString matrixCfg.httpPort}";
@@ -935,10 +944,10 @@ in
     };
 
     networking.firewall = lib.mkIf cfg.openFirewall {
-      allowedTCPPorts =
+      allowedTCPPorts = [
-        [ cfg.port ]
+        cfg.port
-        ++ lib.optional (cfg.selfSignedTls || cfg.tls.certDir != null || cfg.tls.acme.enable)
+      ]
-          cfg.httpsPort;
+      ++ lib.optional (cfg.selfSignedTls || cfg.tls.certDir != null || cfg.tls.acme.enable) cfg.httpsPort;
     };
 
     # `/etc/hosts` entries for local dev — bare hive domain + any