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>

cmd/qfs/main.go

@@ -1152,6 +1152,8 @@ func setupRouter(cfg *config.Config, local *localdb.LocalDB, connMgr *db.Connect
 					"current_version": currentVersion,
 				})
 			})
+
+			configs.GET("/:uuid/export", exportHandler.ExportConfigCSV)
 		}
 
 		projects := api.Group("/projects")