Add Migration Tool to Retroactively Deduplicate Existing Vaults

[S4tvara]

Describe the feature
Introduce a migration tool to retroactively deduplicate existing vaults. Currently, when deduplication is enabled on an existing vault, previously stored chunks are not deduplicated.

Why is this feature needed?

• Existing vaults cannot benefit from deduplication without re-adding files.
• Users waste storage space since duplicate chunks remain untouched.
• Improves consistency with new vaults that already have dedup enabled.

Proposed solution

• Add a sietch dedup migrate command.
• This command should scan existing chunks in the vault, compute their hashes, and update the deduplication index.
• Duplicate chunks should be consolidated and properly tracked in the index.

Alternatives considered

• Manually re-adding files after enabling dedup, which is time-consuming and inefficient.

Additional context

• The limitation is already noted in the code at cmd/dedup.go:124-126.
• Providing a migration command would ensure backward compatibility and help existing users save space without manual intervention.

[srikary12]

Would like to work on this