typing: improve type safety in utils.py, application.py and models.py.

[ExeWall]

Description

This pull request introduces type annotations and strengthens type safety across three core modules of the Vorta codebase to improve maintainability and prevent runtime errors.

Changes in detail:

• src/vorta/utils.py: Added type hints to the search function and FilePathInfoAsync class. Improved logic to handle optional callables correctly.
• src/vorta/application.py: Resolved NoneType vulnerabilities by adding defensive checks (type narrowing) before path operations and .as_uri() calls.
• src/vorta/store/models.py: Fixed an incompatible type assignment between SQLAlchemy StringExpression and standard str by explicitly casting the pattern to a string.

Related Issue

This is a proactive improvement to address static analysis warnings and align with the project's move toward gradual typing. It also serves as my initial contribution for the GSoC 2026 application.

Motivation and Context

The main goal is to improve code robustness. By resolving these Mypy-reported issues, we prevent potential crashes (especially AttributeError on NoneType) that could occur if environment-dependent directories or database expressions are not handled safely.

How Has This Been Tested?

The changes were verified using Mypy 1.x in a local environment:

• Ran mypy src/vorta/utils.py -> Success.
• Ran mypy src/vorta/application.py -> Success.
• Ran mypy src/vorta/store/models.py -> Success.
• Environment: Python 3.10+ on Windows 11.
• Regression: Verified that existing backup and search functionalities remain unaffected by the typing changes.

Types of changes

• 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 change)

Checklist:

• I have read the CONTRIBUTING guide.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

I provide my contribution under the terms of the license of this repository and I affirm the Developer Certificate of Origin.

[ExeWall]

@m3nu can you review when you have time? Thank you so much.

[ExeWall]

@m3nu, hiii, can you review when you have time? Thank you so much.