Address all golangci-lint v2.10.1 (CI's version) findings:
* Add http.MaxBytesReader hard cap to ParseMultipartForm sites in
rest_private.savePictureCtrl (32MB) and api/migrator (256MB) — fixes
G120 by bounding total request body before form parsing.
* Suppress G70x in CLI subcommands cmd/{backup,cleanup,import,remap}.go:
all four issue HTTP requests against operator-supplied RemarkURL/CLI
flags, never user input. Each suppression carries a one-line reason.
* Suppress G122 in image fs_store cleanup walk: staging directory tree
is server-only, no untrusted symlinks land there.