refactor: move backend files to backend/ directory

Reorganize project structure:
- backend/cmd/openteam/ — entry point
- backend/internal/ — core packages
- backend/middleware/ — HTTP middleware
- backend/router/ — route setup
- backend/wire/ — dependency injection
- backend/pkg/ — shared utilities
- backend/go.mod, go.sum — Go module files

Updated Makefile to work from backend/ directory.
Removed old lowercase makefile.

backend/internal/pkg/crypto/crypto_test.go

@@ -0,0 +1,55 @@
+package crypto
+
+import (
+	"testing"
+)
+
+func TestEncryptDecrypt(t *testing.T) {
+	plaintext := "sk-test-api-key-12345"
+
+	encrypted, err := Encrypt(plaintext)
+	if err != nil {
+		t.Fatalf("Encrypt() error = %v", err)
+	}
+
+	if encrypted == plaintext {
+		t.Error("Encrypt() returned plaintext")
+	}
+
+	decrypted, err := Decrypt(encrypted)
+	if err != nil {
+		t.Fatalf("Decrypt() error = %v", err)
+	}
+
+	if decrypted != plaintext {
+		t.Errorf("Decrypt() = %q, want %q", decrypted, plaintext)
+	}
+}
+
+func TestSha256Hex(t *testing.T) {
+	input := "test"
+	result := Sha256Hex(input)
+
+	if len(result) != 64 {
+		t.Errorf("Sha256Hex() returned %d chars, want 64", len(result))
+	}
+
+	// Same input should produce same hash
+	result2 := Sha256Hex(input)
+	if result != result2 {
+		t.Error("Sha256Hex() not deterministic")
+	}
+
+	// Different input should produce different hash
+	result3 := Sha256Hex("different")
+	if result == result3 {
+		t.Error("Sha256Hex() same hash for different inputs")
+	}
+}
+
+func TestEncryptorInvalidKey(t *testing.T) {
+	_, err := NewEncryptor([]byte("short"))
+	if err == nil {
+		t.Error("NewEncryptor() should error with invalid key length")
+	}
+}