Replace a saved template's picture

Upload the image a saved workspace template's card shows (multipart). Owner-or-workspace-admin only, and curated rows are never writable here — the same rule the rename door applies. PNG, JPEG or WebP, read from the file's own magic bytes rather than its name, up to 1 MB. Replaces whatever was there; the content hash moves, so every card pointing at the old picture points somewhere new on its next read.