refactor(agent): stop embedding GeoIP MMDB in agent binary

Agent ships without City/Country MMDB in the binary; Docker images COPY
databases into data_dir, bare installs seed via download on first start.
Server keeps Country-only embed for optional MaxMind control-plane use.
Also harden fetch script nonempty check and reject non-file MMDB paths.

internal/apps/openflare/geoip/data/data_test.go

@@ -0,0 +1,25 @@
+package data
+
+import (
+	"io/fs"
+	"strings"
+	"testing"
+
+	"github.com/oschwald/maxminddb-golang"
+)
+
+func TestEmbeddedCountryDatabaseIsValid(t *testing.T) {
+	raw, err := fs.ReadFile(FS, DefaultMMDBName)
+	if err != nil {
+		t.Fatalf("read embedded Country database: %v", err)
+	}
+	reader, err := maxminddb.FromBytes(raw)
+	if err != nil {
+		t.Fatalf("open embedded Country database: %v", err)
+	}
+	defer reader.Close()
+
+	if !strings.Contains(reader.Metadata.DatabaseType, "Country") {
+		t.Fatalf("unexpected database type %q", reader.Metadata.DatabaseType)
+	}
+}