fix(snapshot): migrate objects from pre-v1.3.3 flat snapshot layout

[criticalcognition]

Issue for this PR

Closes#19437

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

PR #19163 changed the snapshot gitdir from <data>/snapshot/<projectID>/ to <data>/snapshot/<projectID>/<Hash.fast(worktree)>/ but didn't migrate existing objects. Sessions created before v1.3.3 have step-start/step-finish hashes pointing at tree objects in the old directory. When diffFull() tries to diff across those hashes, git returns fatal: bad object and the Review tab shows nothing.

This PR sets up a git alternates file when both old and new snapshot repos exist, allowing git to read objects from the old location transparently. The migration runs once during snapshot service initialization and is safe (errors are caught silently).

Note for users: Sessions affected by this bug will automatically work after this fix is deployed. The Review tab may appear empty initially due to cached summary_files=0 in the database. This will self-heal when:

• The session receives a new message (triggers diff recomputation), or
• The user restarts the desktop app (clears frontend cache)

No manual intervention or data migration is required.

How did you verify your code works?

1. Tested on a local machine with multiple sessions affected by this bug (portal-service, x-portal projects)
2. Verified git alternates setup allows git diff between old and new snapshot hashes
3. Confirmed Review tabs populate correctly after app restart
4. Tested across 11 affected sessions in 3 different projects

Screenshots / recordings

N/A — backend-only change, no UI modifications.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

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[rekram1-node]

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