fix(tw): isolate Taskwarrior data with TASKDATA and serialize CLI ops

[NaitikVerma6776]

Summary

• Stop using the shared /root/.task database: task now runs with TASKDATA and TASKRC inside each request temp dir (cmd.Dir alone was not enough).
• Remove rm -rf /root/.task from fetch/add/complete/delete/modify helpers.
• Serialize all Taskwarrior CLI helpers with a mutex so GET /tasks and mutation jobs cannot overlap.
• Log the real fetch error on GET /tasks; return [] instead of 500 when the task list is empty.

Fixes #471

Test plan

• go test ./utils/ ./utils/tw/ (Taskwarrior tests still need task on PATH)
• In the backend container: after a fetch/add, confirm TASKDATA is the temp dir (not /root/.task) and /root/.task is not deleted as part of the request
• Overlap GET /tasks with an in-flight add/edit; sync should not fail with SQLite 1032 from a concurrent wipe
• Empty task list still returns 200 with []
• Failed fetch still returns 500 and logs the underlying error

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