BUG: [Audit] Backend: Inconsistent password hashing and missing DB rollback logic

### Is there an existing issue for this?

- [x] I have searched the existing issues

### What happened?

During a preliminary technical audit of the PictoPy backend for GSoC 2026, I identified two logical issues that could affect system stability:

Password Consistency: Empty strings are currently being hashed in some paths instead of being treated as None.

Transaction Integrity: The metadata write flow lacks a rollback() mechanism, which could lead to partial data writes if an execution fails.

I have already researched and prepared a fix for these issues to ensure production-grade reliability.

Note: I previously opened PR #1242 with the fixes. I am opening this issue now to follow the proper contribution workflow as requested by the mentor.

MyPy Scan Before Fixes (54 Errors):

MyPy Scan After Fixes (49 Errors):

### Record

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