[previews] Add a command to normalize annotation times

[frankrousseau]

Problem

• Older Kitsu versions stored unrounded annotation times (sometimes as strings), while current ones snap them to the frame grid, so the same logical frame can exist several times in a preview file's annotation list.
• The player only displays the first entry matching a frame, making the other entries' drawings invisible, and grid-timed deletions or updates never match the legacy entries.

Solution

• Add normalize_annotation_times() to merge same-frame entries (objects deduplicated by id) and snap times onto the player's frame grid, mirroring the client-side rounding.
• Add a zou normalize-annotation-times CLI command (per-project filter, --dry-run) to clean existing preview files.