Fix status filtering & persistence bugs, add search filter

[moldscraper23-lab]

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🚀 Title
Fix task filtering & persistence bugs, add case-insensitive search

📝 Summary of Changes
This PR addresses two existing bugs in the FastAPI application and adds a requested search feature for the /tasks endpoint.

🐛 Bug Fixes
Status Filtering (GET /tasks?status=...): Fixed issue where the status query parameter was not properly filtering tasks. Updated logic to filter results by matching status (open / done).

Task Completion Persistence (POST /tasks/{id}/complete): Fixed issue where completing a task modified state in-memory without persisting. Updated the handler to record completed_at timestamp and call store.save_tasks() to persist state to data/tasks.json.

✨ New Features
Task Search (GET /tasks?q=...): Added an optional q query parameter to support case-insensitive matching across task title and description.

Filter Composition: Ensured q and status query parameters compose smoothly together (e.g., GET /tasks?status=open&q=plan).

🧪 Verification & Testing
Tested locally via curl requests against running uvicorn instance:

Bash

1. Status Filter

curl "http://127.0.0.1:8000/tasks?status=open"

2. Persistence Check

curl -X POST "http://127.0.0.1:8000/tasks/3/complete"
curl "http://127.0.0.1:8000/tasks/3"

3. Search Matching

curl "http://127.0.0.1:8000/tasks?q=launch"

4. Composed Query

curl "http://127.0.0.1:8000/tasks?status=open&q=plan"
🧹 Clean Diff Check
Cleaned up runtime modifications to data/tasks.json so the PR contains only application code changes inside app/.