flake: split mediaserver onto nixos-25.11, keep desktops on unstable

The mediaserver kept hard-freezing on local builds (gnupg, openldap,
deno/rusty-v8) whenever a fresh unstable revision outran Hydra's
binary cache. It doesn't need bleeding-edge packages — every service
it runs is mature enough that 6-month-old versions are fine — so move
it onto the stable channel where the cache is essentially always
warm. Gaming and Macbook stay on unstable for fresh GPU/kernel work.

Implementation: add nixpkgs-stable + home-manager-stable inputs,
parameterise mkHost to accept a (nixpkgs, home-manager) pair.

Drive-by:
- Switch homepage.nix from environmentFiles (plural, unstable-only)
  to environmentFile (singular, present on both channels).
- Gate the openldap-skip-tests overlay to non-mediaserver hosts so
  it doesn't force a local rebuild on stable, where openldap is
  always cached.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

flake.nix

@@ -1,11 +1,21 @@
 {
   description = "FredOS NixOS configuration";
   inputs = {
+    # Unstable: gaming desktop & laptop want bleeding-edge GPU/kernel updates.
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    # Stable: mediaserver values cache hit-rate over fresh packages so it
+    # doesn't have to compile gnupg/openldap/v8 locally on every flake bump.
+    nixpkgs-stable.url = "github:NixOS/nixpkgs/nixos-25.11";
+
     home-manager = {
       url = "github:nix-community/home-manager";
       inputs.nixpkgs.follows = "nixpkgs";
     };
+    home-manager-stable = {
+      url = "github:nix-community/home-manager/release-25.11";
+      inputs.nixpkgs.follows = "nixpkgs-stable";
+    };
+
     zen-browser = {
       url = "github:0xc000022070/zen-browser-flake";
       inputs = {
@@ -13,27 +23,37 @@
         home-manager.follows = "home-manager";
       };
     };
-  
+
     nix-cachyos-kernel.url = "github:xddxdd/nix-cachyos-kernel/release";
   };
-  outputs = { self, nixpkgs, home-manager, zen-browser, nix-cachyos-kernel, ... } @ inputs:
-  let
-    system = "x86_64-linux";
-    mkHost = hostname: nixpkgs.lib.nixosSystem {
-      inherit system;
-      specialArgs = { inherit inputs; };
-      modules = [
-        ./hosts/${hostname}.nix
-        ./hosts/hardware/${hostname}.nix 
-        ./common.nix
-        home-manager.nixosModules.home-manager
-      ];
+  outputs =
+    { self
+    , nixpkgs
+    , nixpkgs-stable
+    , home-manager
+    , home-manager-stable
+    , zen-browser
+    , nix-cachyos-kernel
+    , ...
+    } @ inputs:
+    let
+      system = "x86_64-linux";
+      mkHost = hostname: pkgsInput: hmInput: pkgsInput.lib.nixosSystem {
+        inherit system;
+        specialArgs = { inherit inputs; };
+        modules = [
+          ./hosts/${hostname}.nix
+          ./hosts/hardware/${hostname}.nix
+          ./common.nix
+          hmInput.nixosModules.home-manager
+        ];
+      };
+    in
+    {
+      nixosConfigurations = {
+        FredOS-Gaming      = mkHost "FredOS-Gaming"      nixpkgs        home-manager;
+        FredOS-Mediaserver = mkHost "FredOS-Mediaserver" nixpkgs-stable home-manager-stable;
+        FredOS-Macbook     = mkHost "FredOS-Macbook"     nixpkgs        home-manager;
+      };
     };
-  in {
-    nixosConfigurations = {
-      FredOS-Gaming      = mkHost "FredOS-Gaming";
-      FredOS-Mediaserver = mkHost "FredOS-Mediaserver";
-      FredOS-Macbook     = mkHost "FredOS-Macbook";
-    };
-  };
 }