Add --no-lock flag to snapshot indexing operations

[Copilot]

When clicking "Index Snapshots" in the UI, Backrest calls restic snapshots to retrieve repository snapshot history. This operation could fail if another process had a lock on the repository, causing indexing operations to be blocked unnecessarily.

This PR adds the --no-lock flag to snapshot indexing operations, which performs a read-only operation that won't conflict with other processes using the repository.

Changes Made

• Modified RepoOrchestrator.Snapshots() to accept variadic restic.GenericOption parameters for flexibility
• Updated indexSnapshotsHelper() to pass the --no-lock flag when calling repo.Snapshots()
• Added test coverage to verify the --no-lock flag functionality works correctly
• Maintained backward compatibility for all existing callers

Technical Details

The change modifies the snapshot indexing flow:

• IndexSnapshots API → NewOneoffIndexSnapshotsTask → indexSnapshotsHelper → repo.Snapshots(ctx, restic.WithFlags("--no-lock"))

This ensures that snapshot indexing operations use restic snapshots --json --no-lock instead of restic snapshots --json, preventing lock conflicts during repository indexing while other backup or maintenance operations are running.

Fixes #664.

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[garethgeorge]

Please don't pass this through, make it the default in the orchestrator. There probably isn't a scenario where we'd want to do this without --no-lock.

[garethgeorge]

Please address.