feat: add GeoIP country lookup with embedded DB-IP Lite database (PLAN.md 4.3)

Embeds a DB-IP Lite country MMDB (~5MB) in the binary via go:embed,
keeping the single-binary deployment story clean. Country codes are
stored alongside login attempts and sessions, shown in the dashboard
(Top IPs, Top Countries card, Recent/Active Sessions, session detail).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

flake.nix

@@ -18,14 +18,22 @@
           pkgs = nixpkgs.legacyPackages.${system};
           mainGo = builtins.readFile ./cmd/oubliette/main.go;
           version = builtins.head (builtins.match ''.*const Version = "([^"]+)".*'' mainGo);
+          geoipDb = pkgs.fetchurl {
+            url = "https://download.db-ip.com/free/dbip-country-lite-2026-02.mmdb.gz";
+            hash = "sha256-xmQZEJZ5WzE9uQww1Sdb8248l+liYw46tjbfJeu945Q=";
+          };
         in
         {
           default = pkgs.buildGoModule {
             pname = "oubliette";
             inherit version;
             src = ./.;
-            vendorHash = "sha256-smMg/J1igSoSBkzdm9HJOp5OYY8MEccodCD/zVK31IQ=";
+            vendorHash = "sha256-/zxK6CABLYBNtuSOI8dIVgMNxKiDIcbZUS7bQR5TenA=";
             subPackages = [ "cmd/oubliette" ];
+            nativeBuildInputs = [ pkgs.gzip ];
+            preBuild = ''
+              gunzip -c ${geoipDb} > internal/geoip/dbip-country-lite.mmdb
+            '';
             meta = {
               description = "SSH honeypot";
               mainProgram = "oubliette";