Refactor flake targets into platform x build-type matrix

Generates all nixosConfigurations from mkTarget(platform, buildType,
hostPath) instead of hand-written per-host blocks, so adding a new
platform or build type is a one-line addition. Per-machine identity
(hostname, hostId, secrets, stateVersion) moves into hosts/<name>/host.nix;
platform-specific config (hardware, boot, networking) into
modules/platforms/*.nix; build-type config (minimal/server/docker/gui/
nix-cache/pxe-boot) into modules/build-types/*.nix.

Old flat targets (nixos, docker, server, nix-cache, nix-minimal, pxe-boot)
are replaced by the 17-target <platform>-<buildtype> matrix; each new
target was verified to evaluate before its old counterpart was removed.
CI workflows and docs/aliases now discover hosts dynamically via
nixosConfigurations attrNames and /etc/flake-target instead of hardcoded
lists, so they can't drift from flake.nix again.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

modules/build-types/server.nix

@@ -0,0 +1,28 @@
+{ ... }:
+
+{
+  imports = [
+    ../beszel/enable-agent.nix
+    ../services/zfs/enable-service.nix
+  ];
+
+  boot.zfs.extraPools = [ "tank" ];
+
+  systemd.services.nfs-server = {
+    after = [ "zfs-mount.service" ];
+    requires = [ "zfs-mount.service" ];
+  };
+
+  services.nfs.server = {
+    enable = true;
+    exports = ''
+      /tank/docker/config 192.168.2.0/24(rw,sync,no_subtree_check,no_root_squash)
+      /tank/docker/volumes 192.168.2.0/24(rw,sync,no_subtree_check,no_root_squash)
+      /tank/docker/databases 192.168.2.0/24(rw,sync,no_subtree_check,no_root_squash)
+      /tank/docker/nextcloud-data 192.168.2.0/24(rw,sync,no_subtree_check,no_root_squash)
+      /tank/raspi/volumes 192.168.2.0/24(rw,sync,no_subtree_check,no_root_squash)
+    '';
+  };
+
+  networking.firewall.allowedTCPPorts = [ 111 2049 ];
+}