Fix task bugs and add search filter

[Marianne-90]

Summary

• Fix GET /tasks status filtering so it compares against the requested status.
• Persist completed tasks before returning them.
• Add case-insensitive q filtering across task titles and descriptions.
• Preserve existing list behavior when q is omitted and allow q to compose with status.

Root causes

The status filter compared records with the literal string "status", and task completion returned a copied record without updating or saving the stored task list. The route already accepted q, but the service layer did not apply it.

Validation

• Verified open and done status filtering.
• Verified completion persistence and non-null completed_at.
• Verified title and description search with case-insensitive queries.
• Verified combined status and q filtering.
• Ran service-level assertions successfully.
• Confirmed data/tasks.json is not included in the commit.