feat(gateway): assert hsts.enable requires TLS

Addresses mara's review: add a NixOS assertion that rejects
hsts.enable = true when no TLS mode is configured. HSTS over
plain HTTP is silently ignored by browsers; the assertion turns
a silent misconfiguration into a build-time error.

nix/modules/hive-gateway.nix

@@ -331,6 +331,48 @@ in
       };
     };
 
+    hsts = {
+      enable = lib.mkOption {
+        type = lib.types.bool;
+        default = false;
+        description = ''
+          Add `Strict-Transport-Security` to all gateway vhosts.
+
+          Disabled by default: HSTS pins HTTPS in the browser's HSTS
+          preload list; enabling it on a deployment that later loses TLS
+          will lock browsers out until the max-age expires. Only enable
+          this when you are certain TLS is permanent.
+
+          Requires TLS to be active (`selfSignedTls = true`, a `tls.certDir`,
+          or `tls.acme.enable = true`). Enabling HSTS without TLS is
+          technically harmless (browsers ignore the header over plain HTTP)
+          but is almost certainly a misconfiguration.
+        '';
+      };
+
+      maxAge = lib.mkOption {
+        type = lib.types.ints.positive;
+        default = 31536000;
+        example = 86400;
+        description = ''
+          Value for the `max-age` directive in seconds.
+          Default: 31536000 (1 year), which is the value required for
+          HSTS preload list submission. Use a shorter value (e.g. 86400)
+          while testing so browsers forget the pin quickly.
+        '';
+      };
+
+      includeSubDomains = lib.mkOption {
+        type = lib.types.bool;
+        default = true;
+        description = ''
+          Whether to include `includeSubDomains` in the HSTS header.
+          Only disable this if the gateway host has sub-domains that
+          intentionally serve plain HTTP.
+        '';
+      };
+    };
+
   };
 
   config = lib.mkIf cfg.enable {
@@ -375,6 +417,18 @@ in
           Let's Encrypt needs a contact address for the ACME account.
         '';
       }
+      {
+        assertion =
+          !cfg.hsts.enable
+          || cfg.selfSignedTls
+          || cfg.tls.certDir != null
+          || cfg.tls.acme.enable;
+        message = ''
+          services.hyperhive.gateway.hsts.enable = true requires TLS to be
+          configured (selfSignedTls, tls.certDir, or tls.acme.enable). HSTS
+          over plain HTTP is ignored by browsers and indicates a config error.
+        '';
+      }
     ];
 
     # Ensure bind-mount sources exist at host boot before the gateway
@@ -444,15 +498,9 @@ 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
-              "/run/hive-tls/${cfg.tls.certName}"
-            else
-              "${tlsDir}/cert.pem";
+            if cfg.tls.certDir != null then "/run/hive-tls/${cfg.tls.certName}" else "${tlsDir}/cert.pem";
           tlsKey =
-            if cfg.tls.certDir != null then
-              "/run/hive-tls/${cfg.tls.keyName}"
-            else
-              "${tlsDir}/key.pem";
+            if cfg.tls.certDir != null then "/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 +546,23 @@ in
           publicPort = if hasTls then cfg.httpsPort else cfg.port;
           publicPortDefault = if hasTls then 443 else 80;
           publicPortSuffix = if publicPort == publicPortDefault then "" else ":${toString publicPort}";
+
+          # Security headers added at the server scope on every vhost.
+          # nginx's add_header inheritance rule: a location that defines its
+          # own add_header does NOT inherit the server-level ones, so API
+          # locations with CORS headers (e.g. /.well-known/matrix/client,
+          # /_matrix/) are unaffected. HTML-serving and proxy locations that
+          # carry no add_header of their own pick these up automatically.
+          hstsDirectives = lib.concatStringsSep "; " (
+            [ "max-age=${toString cfg.hsts.maxAge}" ]
+            ++ lib.optional cfg.hsts.includeSubDomains "includeSubDomains"
+          );
+          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;
+            ${lib.optionalString cfg.hsts.enable ''add_header Strict-Transport-Security "${hstsDirectives}" always;''}
+          '';
         in
         {
           system.stateVersion = "26.05";
@@ -794,7 +859,7 @@ 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 +873,7 @@ 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 +896,7 @@ 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 +1002,10 @@ in
     };
 
     networking.firewall = lib.mkIf cfg.openFirewall {
-      allowedTCPPorts =
-        [ cfg.port ]
-        ++ lib.optional (cfg.selfSignedTls || cfg.tls.certDir != null || cfg.tls.acme.enable)
-          cfg.httpsPort;
+      allowedTCPPorts = [
+        cfg.port
+      ]
+      ++ lib.optional (cfg.selfSignedTls || cfg.tls.certDir != null || cfg.tls.acme.enable) cfg.httpsPort;
     };
 
     # `/etc/hosts` entries for local dev — bare hive domain + any