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/service/agent.go

@@ -21,7 +21,7 @@ type AgentService struct {
 	targetRepo     repository.TargetRepository
 	profileRepo    repository.CertificateProfileRepository
 	auditService   *AuditService
-	issuerRegistry map[string]IssuerConnector
+	issuerRegistry *IssuerRegistry
 	renewalService *RenewalService
 }
 
@@ -32,7 +32,7 @@ func NewAgentService(
 	jobRepo repository.JobRepository,
 	targetRepo repository.TargetRepository,
 	auditService *AuditService,
-	issuerRegistry map[string]IssuerConnector,
+	issuerRegistry *IssuerRegistry,
 	renewalService *RenewalService,
 ) *AgentService {
 	return &AgentService{
@@ -163,7 +163,7 @@ func (s *AgentService) SubmitCSR(ctx context.Context, agentID string, certID str
 		}
 
 		// Fallback: direct issuer signing (no AwaitingCSR job — ad-hoc CSR submission)
-		connector, ok := s.issuerRegistry[cert.IssuerID]
+		connector, ok := s.issuerRegistry.Get(cert.IssuerID)
 		if ok {
 			// Resolve EKUs from the certificate profile if available
 			var ekus []string