Fix task bugs and add search filter

[Akram012388]

What changed

• fix GET /tasks status filtering
• add case-insensitive q search across task titles and descriptions
• make status and search filters compose correctly
• persist completed task state to the JSON store
• add isolated HTTP-level regression tests
• add GitHub Actions coverage for Python 3.9 and 3.14

Why

The status filter compared records with the literal string "status", the accepted q parameter was not applied, and task completion returned an updated copy without saving it. These changes implement the documented API behavior and protect it with automated tests that do not modify the sample data.

Python 3.9 also needs eval-type-backport to evaluate the project's modern Pydantic annotations, so it is installed conditionally to preserve the documented Python 3.9+ support range.

Verification

• python -m pytest -W error -v on Python 3.9: 6 passed
• python -m pytest -W error -v on Python 3.14: 6 passed
• python -m pip check
• python -m compileall -q app tests
• git diff --check
• confirmed data/tasks.json is unchanged

Closes #2
Closes #3
Closes #4
Closes #5