Fix task bugs and add search filter

[AntonKruhlou]

Summary

Completes the CodexLab Task Tracker lab: fixes both intentional bugs and adds the search feature.

• Fix Bug A: GET /tasks?status=... compared against the literal string "status" instead of the query parameter, so any status filter returned an empty list. Now compares against the actual status value.
• Fix Bug B: POST /tasks/{id}/complete built the updated task but never wrote it back to the in-memory list or called save_tasks, so completion never persisted. Now updates the list in place and saves it.
• Add feature: q query parameter on GET /tasks does a case-insensitive substring search across title and description, composable with status, and is a no-op when omitted.

Verification

• curl "/tasks?status=open" / ?status=done each return only matching tasks
• POST /tasks/3/complete then GET /tasks/3 shows status: done with a non-null completed_at
• q=launch, q=report, q=LAUNCH (case-insensitive) all match expected tasks
• status=open&q=launch combines both filters correctly
• data/tasks.json restored to its original committed state before this PR (no runtime test data included)