export: implement streaming CSV with Excel compatibility

Implement Phase 1 CSV Export Optimization:
- Replace buffering with true HTTP streaming (ToCSV writes to io.Writer)
- Add UTF-8 BOM (0xEF 0xBB 0xBF) for correct Cyrillic display in Excel
- Use semicolon (;) delimiter for Russian Excel locale
- Use comma (,) as decimal separator in numbers (100,50 instead of 100.50)
- Add graceful two-phase error handling:
  * Before streaming: return JSON errors for validation failures
  * During streaming: log errors only (HTTP 200 already sent)
- Add backward-compatible ToCSVBytes() helper
- Add GET /api/configs/:uuid/export route for configuration export

New tests (13 total):
- Service layer (7 tests):
  * UTF-8 BOM verification
  * Semicolon delimiter parsing
  * Total row formatting
  * Category sorting
  * Empty data handling
  * Backward compatibility wrapper
  * Writer error handling
- Handler layer (6 tests):
  * Successful CSV export with streaming
  * Invalid request validation
  * Empty items validation
  * Config export with proper headers
  * 404 for missing configs
  * Empty config validation

All tests passing, build verified.

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

internal/handlers/export.go

@@ -47,15 +47,22 @@ func (h *ExportHandler) ExportCSV(c *gin.Context) {
 
 	data := h.buildExportData(&req)
 
-	csvData, err := h.exportService.ToCSV(data)
-	if err != nil {
-		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
+	// Validate before streaming (can return JSON error)
+	if len(data.Items) == 0 {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "no items to export"})
 		return
 	}
 
+	// Set headers before streaming
 	filename := fmt.Sprintf("%s %s SPEC.csv", time.Now().Format("2006-01-02"), req.Name)
+	c.Header("Content-Type", "text/csv; charset=utf-8")
 	c.Header("Content-Disposition", fmt.Sprintf("attachment; filename=\"%s\"", filename))
-	c.Data(http.StatusOK, "text/csv; charset=utf-8", csvData)
+
+	// Stream CSV (cannot return JSON after this point)
+	if err := h.exportService.ToCSV(c.Writer, data); err != nil {
+		c.Error(err) // Log only
+		return
+	}
 }
 
 func (h *ExportHandler) buildExportData(req *ExportRequest) *services.ExportData {
@@ -101,6 +108,7 @@ func (h *ExportHandler) ExportConfigCSV(c *gin.Context) {
 	username := middleware.GetUsername(c)
 	uuid := c.Param("uuid")
 
+	// Get config before streaming (can return JSON error)
 	config, err := h.configService.GetByUUID(uuid, username)
 	if err != nil {
 		c.JSON(http.StatusNotFound, gin.H{"error": err.Error()})
@@ -109,13 +117,20 @@ func (h *ExportHandler) ExportConfigCSV(c *gin.Context) {
 
 	data := h.exportService.ConfigToExportData(config, h.componentService)
 
-	csvData, err := h.exportService.ToCSV(data)
-	if err != nil {
-		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
+	// Validate before streaming (can return JSON error)
+	if len(data.Items) == 0 {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "no items to export"})
 		return
 	}
 
+	// Set headers before streaming
 	filename := fmt.Sprintf("%s %s SPEC.csv", config.CreatedAt.Format("2006-01-02"), config.Name)
+	c.Header("Content-Type", "text/csv; charset=utf-8")
 	c.Header("Content-Disposition", fmt.Sprintf("attachment; filename=\"%s\"", filename))
-	c.Data(http.StatusOK, "text/csv; charset=utf-8", csvData)
+
+	// Stream CSV (cannot return JSON after this point)
+	if err := h.exportService.ToCSV(c.Writer, data); err != nil {
+		c.Error(err) // Log only
+		return
+	}
 }