feat(05-02): implement FilesDeps + FileUploadHandler + TabloFilesTabHandler + TabloTasksTabHandler
- FilesDeps struct with Queries, Files FileStorer, MaxUploadMB
- loadOwnedTabloForFile helper (mirrors loadOwnedTabloForTask)
- TabloFilesTabHandler: nil guard first, loadOwnedTablo, list files, HTMX/full-page dispatch
- TabloTasksTabHandler: loadOwnedTablo, list tasks, HTMX/full-page dispatch
- FileUploadHandler: nil guard, MaxBytesReader before ParseMultipartForm, S3 key files/{uuid}, InsertTabloFile, list + redirect
- FileDownloadHandler/FileDeleteConfirmHandler/FileDeleteHandler: 501 stubs for Plan 03
- Security: D-04 S3 key isolation, T-05-02-02 size guard, T-05-02-04 ownership