Add workflow export bundle endpoint and deterministic serialization

[akgohain]

Motivation

• Provide a backend endpoint to export a reproducible workflow bundle containing session snapshot, ordered events, and referenced artifact paths with existence flags.
• Make exported bundles deterministic (stable schema version and ordering) and safe when referenced files are missing.

Description

• Add a new router with POST /api/workflows/{workflow_id}/export-bundle that returns a typed WorkflowExportBundle response model (server_api/workflow/router.py).
• Introduce Pydantic models for the bundle payload including WorkflowExportBundle, WorkflowEvent, and WorkflowArtifactEntry (server_api/workflow/models.py).
• Implement build_export_bundle in server_api/workflow/service.py which sorts events chronologically (by timestamp then id), discovers artifact paths from the session snapshot and events when artifact_paths is not provided, and produces exists: true/false per artifact without raising on missing files.
• Mount the workflow router in the main app by including it in server_api/main.py and add focused unit tests (tests/test_workflow_export_bundle.py) that cover the happy path and explicit artifact path / missing-file behavior.

Testing

• Ran PYTHONPATH=. pytest -q tests/test_workflow_export_bundle.py which executed the new tests and produced 2 passed.
• Attempted uv run pytest -q tests/test_workflow_export_bundle.py which failed in this environment due to an unrelated editable-package metadata issue for pytorch_connectomics (packaging error), not the new logic.

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Codex Task

[akgohain]

Consolidated into draft PR #165 (feat/codex-wave-integration). Closing this source draft to keep review in one place.