fix: delete the previous content when a file is replaced

[justadityaraj]

Description

Closes #407.

ReplaceFile rewrites the existing metadata entry in place, pointing it at the new content's SHA1, AwsBucket and size. Whatever the file used to hold is then referenced by no entry at all.

CleanUp frees a source by walking metadata: for each entry it considers expired or missing it checks whether any other entry shares the same SHA1, and only then removes the stored data. Since nothing points at the replaced content any more, CleanUp never visits it, so it is never reclaimed. That is why the storage stays occupied whether or not deleteNewFile is set: deleteNewFile only expires the entry holding the new content.

The fix keeps a copy of the entry before it is rewritten and deletes that source afterwards, unless another entry still shares the same SHA1 (a duplicate, or a replace with identical content).

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactor / Chore

Technical Details

• Database changes: No
• Storage backend affected: Both. The deletion goes through the existing deleteSource, which already routes to aws.DeleteObject for non-local files and keeps the old AwsBucket from the copied entry.
• Usage of AI: Yes. AI assisted with tracing the ReplaceFile -> CleanUp path and drafting the test. I confirmed the root cause against the code, decided the approach, and ran the suite myself.

How Has This Been Tested?

• Unit Tests: new TestReplaceFileDeletesPreviousContent writes both sources to the data dir, replaces one with the other, and asserts the previous source is gone while the new one remains. It fails on master (Assertion failed, got: false, want: true) and passes with the change. TestReplaceFile still passes.
• Environment: Windows, go test ./internal/storage/... --tags=test,awsmock.
• Four tests fail identically before and after this change on this host: TestNewFile, TestGetFileByChunkId, TestLocalStorageDriver_Init and TestGetDataPath. They look like path-separator assumptions rather than anything related to this change.
• go vet --tags=test,awsmock ./internal/storage/ and gofmt -l clean.

Checklist

• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.