feat(organize): organize arbitrary source dir (e.g. downloads) with dedup + 洗版

- Add OrganizeDirectory service: walk an arbitrary source directory (download
  dir / NAS direct-read path) and organize video files into the destination,
  without requiring the source to be a registered library.
- Dedup: skip media already present in the destination (matched by scanned DB
  identity title+year[/season+episode], robust to dir case/layout, plus a
  filesystem folder fallback).
- 洗版 (resolution replacement): when the source resolution is higher than the
  existing version, replace the lower-res file (+NFO sidecar +DB row). Prefers
  scanned dimensions, then ffprobe, then filename token; never replaces on
  unknown resolution.
- New endpoints: GET /admin/organize/sources (download/media dir candidates)
  and POST /admin/organize/source.
- UI: ToolsPage adds a '整理来源目录（去重+洗版）' form so operators can pick the
  download dir as the organize source.

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

internal/handler/handler.go

@@ -347,6 +347,8 @@ func Register(r *gin.Engine, cfg *config.Config, log *zap.Logger, svc *service.C
 			// File organizer.
 			admin.POST("/media/:id/organize", organizeMediaHandler(svc))
 			admin.POST("/libraries/:id/organize", organizeLibraryHandler(svc))
+			admin.GET("/organize/sources", organizeSourcesHandler(svc))
+			admin.POST("/organize/source", organizeDirectoryHandler(svc))
 
 			// API key management (encrypted at rest).
 			admin.GET("/api-configs", listAPIConfigsHandler(svc))