Handle missing task deletions gracefully

[seanwevans]

Summary

• make TaskRecoveryManager::remove_task return a boolean and log missing tasks at debug level
• return HTTP 404 from the delete task handler when the task does not exist
• extend the delete task API test to cover the not-found response

Testing

• cargo test (fails: timed out acquiring Postgres connections in existing database-dependent tests)

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https://chatgpt.com/codex/tasks/task_e_68d829dddf948328869a74cdc194f794