feat(M34): dynamic issuer configuration with encrypted config storage

Replace static env-var-based issuer wiring with GUI-driven dynamic
configuration stored encrypted in PostgreSQL. Operators can now
configure, test, enable/disable, and manage issuers from the dashboard
without restarting the server.

Key changes:
- AES-256-GCM encryption for sensitive issuer config at rest (PBKDF2
  key derivation with 100k iterations)
- Dynamic IssuerRegistry with sync.RWMutex replacing static map
- Connector factory pattern (issuerfactory.NewFromConfig) replacing
  140 lines of static wiring in main.go
- Migration 000009: encrypted_config, last_tested_at, test_status,
  source columns on issuers table
- Env var seeding on first boot with ON CONFLICT DO NOTHING
- Registry Rebuild() for atomic map swap after CRUD operations
- Issuer type validation against domain constants on Create
- Audit trail for test connection results
- Conditional seeding for step-ca/OpenSSL (only when env vars set)
- GUI: source badge, connection test status on issuer detail page

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

internal/crypto/encryption.go

@@ -0,0 +1,103 @@
+// Package crypto provides AES-256-GCM encryption for sensitive configuration data.
+package crypto
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"crypto/sha256"
+	"fmt"
+	"io"
+
+	"golang.org/x/crypto/pbkdf2"
+)
+
+// Encrypt encrypts plaintext using AES-256-GCM with a random 12-byte nonce prepended to the output.
+// The key must be exactly 32 bytes (AES-256). Returns [12-byte nonce][ciphertext+tag].
+func Encrypt(plaintext []byte, key []byte) ([]byte, error) {
+	if len(key) != 32 {
+		return nil, fmt.Errorf("encryption key must be exactly 32 bytes, got %d", len(key))
+	}
+
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create AES cipher: %w", err)
+	}
+
+	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create GCM: %w", err)
+	}
+
+	nonce := make([]byte, gcm.NonceSize())
+	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
+		return nil, fmt.Errorf("failed to generate nonce: %w", err)
+	}
+
+	ciphertext := gcm.Seal(nonce, nonce, plaintext, nil)
+	return ciphertext, nil
+}
+
+// Decrypt decrypts ciphertext that was encrypted with Encrypt.
+// Expects format: [12-byte nonce][ciphertext+tag]. Key must be exactly 32 bytes.
+func Decrypt(ciphertext []byte, key []byte) ([]byte, error) {
+	if len(key) != 32 {
+		return nil, fmt.Errorf("encryption key must be exactly 32 bytes, got %d", len(key))
+	}
+
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create AES cipher: %w", err)
+	}
+
+	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create GCM: %w", err)
+	}
+
+	nonceSize := gcm.NonceSize()
+	if len(ciphertext) < nonceSize {
+		return nil, fmt.Errorf("ciphertext too short: %d bytes", len(ciphertext))
+	}
+
+	nonce, ciphertextBody := ciphertext[:nonceSize], ciphertext[nonceSize:]
+	plaintext, err := gcm.Open(nil, nonce, ciphertextBody, nil)
+	if err != nil {
+		return nil, fmt.Errorf("failed to decrypt: %w", err)
+	}
+
+	return plaintext, nil
+}
+
+// DeriveKey derives a 32-byte AES-256 key from a passphrase using PBKDF2-SHA256.
+// Uses a fixed application-specific salt and 100,000 iterations for resistance
+// to brute-force attacks on weak passphrases.
+func DeriveKey(passphrase string) []byte {
+	// Fixed salt is acceptable here because:
+	// 1. Each certctl instance has its own passphrase
+	// 2. The salt prevents generic rainbow table attacks
+	// 3. Per-user salts are unnecessary (single server key, not user passwords)
+	salt := []byte("certctl-config-encryption-v1")
+	return pbkdf2.Key([]byte(passphrase), salt, 100000, 32, sha256.New)
+}
+
+// EncryptIfKeySet encrypts plaintext if a key is provided, otherwise returns plaintext unchanged.
+// This supports the development/demo fallback where encryption isn't configured.
+func EncryptIfKeySet(plaintext []byte, key []byte) ([]byte, bool, error) {
+	if len(key) == 0 {
+		return plaintext, false, nil
+	}
+	encrypted, err := Encrypt(plaintext, key)
+	if err != nil {
+		return nil, false, err
+	}
+	return encrypted, true, nil
+}
+
+// DecryptIfKeySet decrypts ciphertext if a key is provided, otherwise returns ciphertext unchanged.
+func DecryptIfKeySet(ciphertext []byte, key []byte) ([]byte, error) {
+	if len(key) == 0 {
+		return ciphertext, nil
+	}
+	return Decrypt(ciphertext, key)
+}