Enhance notification APIs with HTML email support, CC/BCC, and structured logging

Add comprehensive improvements across REST and gRPC APIs:
  - Add structured logging for all notification operations
  - Implement HTML email support with multipart/alternative MIME
  - Add CC and BCC recipient support for email notifications
  - Add GetNotifiers endpoint to query available notifier configurations
  - Support configurable From name in SMTP configuration
  - Auto-detect content type (text vs HTML) in notification bodies
  - Improve error handling and validation across all endpoints

  🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

internal/domain/notifier.go

@@ -57,6 +57,9 @@ type NotificationService interface {
 
 	// GetStats returns notification statistics
 	GetStats(ctx context.Context) (*NotificationStats, error)
+
+	// GetNotifiers returns information about available notifiers
+	GetNotifiers(ctx context.Context) (*NotifiersResponse, error)
 }
 
 // NotificationStats contains statistics about notification processing
@@ -69,3 +72,15 @@ type NotificationStats struct {
 	ByStatus       map[string]int64 `json:"by_status"`
 	AverageLatency float64          `json:"average_latency_ms"`
 }
+
+// NotifierInfo contains information about a configured notifier type
+type NotifierInfo struct {
+	Type           NotificationType `json:"type"`
+	Accounts       []string         `json:"accounts"`
+	DefaultAccount string           `json:"default_account"`
+}
+
+// NotifiersResponse contains the list of available notifiers
+type NotifiersResponse struct {
+	Notifiers []NotifierInfo `json:"notifiers"`
+}