Add completion-time filter (completed_after/completed_before) to workflow list API #1143

What new feature or improvement would you like to see?

Summary
The workflow list API supports filtering by submission time (submitted_after) but has no equivalent filter for completion time. As a result, any "completed within window X" query has to be approximated by submission time, which is incorrect for long-running workflows. The UI "Completed (24h)" dashboard card is the most visible symptom of this gap.

Current behavior (symptom)

• The "Completed (24h)" dashboard card filters workflows using a submission-time parameter, not a completion-time parameter.
• A workflow submitted >24h ago but completed within the last 24h is excluded from the card, and a workflow submitted within 24h but still running can be miscounted depending on status handling.
• There is no API query parameter to filter by completion time at all, so this cannot be worked around client-side without pulling the full list and filtering locally.

Root cause
The workflow list/query endpoint exposes only submission-time filtering. There is no completed_after / completed_before parameter, and (to confirm) the underlying schema may not expose a queryable, indexed completion timestamp.

Proposed solution

1. API – add completed_after / completed_before query parameters to the workflow list endpoint, mirroring the existing submitted_after semantics (UTC timestamps).
2. DB / query – filter on the workflow completion timestamp. If a queryable completion-time column does not already exist (or is only derivable from a status-transition event), add/expose one with an appropriate index.
3. UI – switch the "Completed (Nh)" dashboard card from the submission-time filter to completed_after.
4. CLI (optional, for consistency) – osmo workflow list --completed-after / --completed-before.

Environment

• OSMO version: <1.2.1>
• Deployment: Kubernetes (EKS), backend orchestration via OSMO

Willing to contribute
Happy to implement this

Why is this needed?

Impact

• Dashboard "Completed (Nh)" metrics are inaccurate for any workload whose runtime exceeds the window — common for training/sim jobs.
• Operators cannot answer "what completed in the last N hours" via API/CLI, which affects reporting, alerting, and capacity review.

Code of Conduct

• I agree to follow NVIDIA OSMO's Code of Conduct
• I have searched the open feature requests and have found no duplicates for this feature request