process scan: set Source on AgentCommand to avoid check constraint crash

TriggerProcessScan was building an AgentCommand without Source, which hit
the agent_commands_source_check constraint every time. Every other
command-creation site sets this correctly; this one got missed.

Thanks to QiTechCo for the catch during v0.2.8.0 dev testing.

Co-Authored-By: Xzaviar <user@QiTechCo.qitc.io>

server/internal/api/handlers/processes.go

@@ -341,6 +341,7 @@ func (h *ProcessHandler) TriggerProcessScan(c *gin.Context) {
 		AgentID:     agentID,
 		CommandType: "scan_processes",
 		Status:      "pending",
+		Source:      "manual",
 		CreatedAt:   time.Now().UTC(),
 		UpdatedAt:   time.Now().UTC(),
 	}