Offer to add workflows to earlier videos, with backups (0.2.3)

[KBRASK]

Fixes #64.

Videos made before v0.2.2 keep their workflow only in workflow.json. This offers to add it to them, safely and only when the user asks. Release 0.2.3.

Asked first

• Nothing runs at startup.
• Creations shows how many earlier videos can get their workflow and the space their backups take, with Add workflows and Don't ask again.
• Adding runs in the background, then reports how many videos changed and where the originals are kept. "Don't ask again" is remembered by the server.
• --disable-metadata hides the offer.

Changes nothing it cannot verify

• A video is skipped when any of these holds:
  • its workflow.json has no canvas workflow (requests queued through the API save an empty one);
  • it already has a workflow;
  • its request is still running;
  • free space is under twice its size plus 512 MiB;
  • it has anything besides audio and video streams;
  • a backup already exists.
• The rewrite replaces the original only after every packet of every stream matches byte for byte and the workflow reads back.
• The original is kept as video.before-workflow.mp4: a hard link, or a size-checked copy.
• The result cache is updated, so saved results stay reusable.
• New videos get the same check. Sharing exports ignore an empty workflow.json.

Checks

• Unit tests: rewriting and skipping, a mismatched rewrite keeping the original, low disk, an existing backup, and the candidate count.
• A browser test for the Creations offer, dismissal and confirmed run, in English and Chinese.
• On the Windows test machine, with copies of real earlier outputs:
  • the video with a canvas workflow was rewritten, and its backup matches the original's SHA-256;
  • dropping it on that ComfyUI's canvas restored the prompt and seed;
  • the two API-queued videos stayed byte-identical, with no backup.