feat: add error codes to genome API responses

Replace generic 'no genome data' message with specific error codes:
- GENOME_NO_EXTRACTION: extraction entry not found
- GENOME_VARIANT_QUERY_FAILED: variant query failed

Makes debugging MCP issues much faster by pinpointing exact failure point.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

api/api_genome.go

@@ -91,7 +91,11 @@ func handleGenomeQuery(w http.ResponseWriter, r *http.Request) {
 	extraction, err := lib.GenomeGetExtraction(sysCtx, dossierID)
 	if err != nil {
 		w.Header().Set("Content-Type", "application/json")
-		json.NewEncoder(w).Encode(map[string]string{"error": "no genome data for this dossier"})
+		json.NewEncoder(w).Encode(map[string]interface{}{
+			"error": "genome extraction entry not found",
+			"code":  "GENOME_NO_EXTRACTION",
+			"detail": err.Error(),
+		})
 		return
 	}
 
@@ -130,7 +134,11 @@ func handleGenomeQuery(w http.ResponseWriter, r *http.Request) {
 	variants, err := lib.GenomeGetVariants(sysCtx, dossierID, tierIDs)
 	if err != nil {
 		w.Header().Set("Content-Type", "application/json")
-		json.NewEncoder(w).Encode(GenomeResponse{Matches: []GenomeMatch{}, Returned: 0, Total: 0})
+		json.NewEncoder(w).Encode(map[string]interface{}{
+			"error": "failed to retrieve genome variants",
+			"code":  "GENOME_VARIANT_QUERY_FAILED",
+			"detail": err.Error(),
+		})
 		return
 	}