🐛 Fix: Resolve 'whenever' library deprecation warnings across codebase

[Vineshnayak]

Description

This PR resolves multiple WheneverDeprecationWarnings that are currently triggered across the codebase by recent updates to the whenever library. By addressing these deprecations, this PR ensures that the test logs remain clean and the project is safe from future breaking changes in the whenever dependency.

Specifically, the following deprecated usages were updated to their modern equivalents:

1. Timezone Configuration: Replaced the deprecated .to_system_tz() method with .to_tz(SYSTEM_TZ) in logging.py and reproducibility.py.
2. Datetime Parsing: Replaced the deprecated format argument with the new pattern argument in PlainDateTime.parse() calls within the Dashboard module (app.py, scanner.py, and test files).
3. Format Specifiers: Updated the deprecated lowercase hh specifier to the standard HH specifier for 24-hour time formatting across logging and reproducibility workflows.
4. TimeDeltas: Replaced the deprecated integer offset in to_fixed_offset(0) with the explicit hours(0) timedelta in the fenn pull CLI command logic (pull.py).

Checklist

• I have read the CONTRIBUTING.md guidelines.
• I ran the test suite locally (uv run pytest tests/) and confirmed that all 919 tests pass successfully.
• I verified that all 77 WheneverDeprecationWarnings have been completely eliminated from the test execution logs.
• I ran the linters (ruff check .) and code formatting checks with zero errors.