fix: allow empty ai_provider in CreateJob — uses tenant settings

Changed AIProvider binding from required to omitempty. When empty,
analyzer falls back to tenant AI settings (already implemented).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: tanviet12

backend/api/handlers/jobs.go

@@ -33,7 +33,7 @@ type CreateJobRequest struct {
 	RulesContent    string          `json:"rules_content"`
 	RulesConfig     json.RawMessage `json:"rules_config"`
 	SkipConditions  string          `json:"skip_conditions"`
-	AIProvider      string          `json:"ai_provider" binding:"required,oneof=claude gemini"`
+	AIProvider      string          `json:"ai_provider" binding:"omitempty,oneof=claude gemini"`
 	AIModel         string          `json:"ai_model"`
 	Outputs         json.RawMessage `json:"outputs" binding:"required"`
 	OutputSchedule  string          `json:"output_schedule" binding:"required,oneof=instant scheduled cron none"`