feat(server): API relay gateway backend M0-M4

Gin + GORM + pure-Go SQLite. Users/auth (JWT), API key management with
quotas, proxy gateway with weighted channel failover and health checks,
usage/billing ledger, cross-protocol conversion (Anthropic Messages /
OpenAI Chat Completions / OpenAI Responses), and channel/model admin API.
Channels declare native API formats and auto-convert the rest.

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

server/internal/pkg/crypto/crypto.go

@@ -0,0 +1,64 @@
+package crypto
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/base64"
+	"errors"
+	"fmt"
+	"io"
+)
+
+// Encrypt seals a secret with AES-GCM. The master key can be any string;
+// it is hashed to a fixed-size AES key.
+func Encrypt(plaintext, masterKey string) (string, error) {
+	key := sha256.Sum256([]byte(masterKey))
+	block, err := aes.NewCipher(key[:])
+	if err != nil {
+		return "", err
+	}
+	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return "", err
+	}
+	nonce := make([]byte, gcm.NonceSize())
+	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
+		return "", err
+	}
+	sealed := gcm.Seal(nonce, nonce, []byte(plaintext), nil)
+	return base64.StdEncoding.EncodeToString(sealed), nil
+}
+
+// Decrypt opens a ciphertext produced by Encrypt.
+func Decrypt(ciphertext, masterKey string) (string, error) {
+	key := sha256.Sum256([]byte(masterKey))
+	data, err := base64.StdEncoding.DecodeString(ciphertext)
+	if err != nil {
+		return "", err
+	}
+	block, err := aes.NewCipher(key[:])
+	if err != nil {
+		return "", err
+	}
+	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return "", err
+	}
+	if len(data) < gcm.NonceSize() {
+		return "", errors.New("ciphertext too short")
+	}
+	nonce, sealed := data[:gcm.NonceSize()], data[gcm.NonceSize():]
+	plain, err := gcm.Open(nil, nonce, sealed, nil)
+	if err != nil {
+		return "", fmt.Errorf("decrypt failed (bad master key?): %w", err)
+	}
+	return string(plain), nil
+}
+
+// HashSHA256 returns the hex SHA-256 of a string (used for API key lookup).
+func HashSHA256(s string) string {
+	sum := sha256.Sum256([]byte(s))
+	return fmt.Sprintf("%x", sum)
+}