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>

api/grpc/notifier.proto

@@ -29,6 +29,9 @@ service NotifierService {
   // GetStats returns notification statistics
   rpc GetStats(GetStatsRequest) returns (GetStatsResponse);
 
+  // GetNotifiers returns information about available notifiers
+  rpc GetNotifiers(GetNotifiersRequest) returns (GetNotifiersResponse);
+
   // HealthCheck verifies the service is operational
   rpc HealthCheck(HealthCheckRequest) returns (HealthCheckResponse);
 }
@@ -51,6 +54,13 @@ enum Priority {
   PRIORITY_CRITICAL = 4;
 }
 
+// ContentType defines the format of the notification body
+enum ContentType {
+  CONTENT_TYPE_UNSPECIFIED = 0;
+  CONTENT_TYPE_TEXT = 1;
+  CONTENT_TYPE_HTML = 2;
+}
+
 // NotificationStatus represents the state of a notification
 enum NotificationStatus {
   NOTIFICATION_STATUS_UNSPECIFIED = 0;
@@ -71,7 +81,10 @@ message Notification {
   NotificationStatus status = 5;
   string subject = 6;
   string body = 7;
+  ContentType content_type = 18; // Format of the body (text or html)
   repeated string recipients = 8;
+  repeated string cc = 16; // Carbon copy recipients (email only)
+  repeated string bcc = 17; // Blind carbon copy recipients (email only)
   map<string, string> metadata = 9;
   google.protobuf.Timestamp created_at = 10;
   google.protobuf.Timestamp scheduled_for = 11;
@@ -98,7 +111,10 @@ message SendNotificationRequest {
   Priority priority = 3;
   string subject = 4;
   string body = 5;
+  ContentType content_type = 12; // Format of the body (text or html) - auto-detected if not specified
   repeated string recipients = 6;
+  repeated string cc = 10; // Carbon copy recipients (email only)
+  repeated string bcc = 11; // Blind carbon copy recipients (email only)
   map<string, string> metadata = 7;
   google.protobuf.Timestamp scheduled_for = 8;
   int32 max_retries = 9;
@@ -187,6 +203,21 @@ message GetStatsResponse {
   double average_latency_ms = 7;
 }
 
+// GetNotifiersRequest requests available notifiers
+message GetNotifiersRequest {}
+
+// NotifierInfo contains information about a configured notifier type
+message NotifierInfo {
+  NotificationType type = 1;
+  repeated string accounts = 2;
+  string default_account = 3;
+}
+
+// GetNotifiersResponse returns available notifiers
+message GetNotifiersResponse {
+  repeated NotifierInfo notifiers = 1;
+}
+
 // HealthCheckRequest requests health status
 message HealthCheckRequest {}