fix: preserve original timestamps on workflow revisions

[DragonBot00]

Summary

Fixes #5328

Problem

In the workflow Versions panel, every revision was displaying the same timestamp — the time of the most-recent save. Older revisions had their updated_at timestamp silently overwritten each time a new revision was created.

Root Cause

WorkflowVersion.updated_at was defined with onupdate=func.now() in the SQLAlchemy model:

updated_at: datetime = Field(
    sa_column=Column(
        DateTime(timezone=True),
        name="updated_at",
        onupdate=func.now(),   # <-- the culprit
        server_default=func.now(),
        nullable=False,
    )
)

When saving a new revision, the code first runs:

session.exec(
    update(WorkflowVersion)
    .where(col(WorkflowVersion.workflow_id) == existing_workflow.id)
    .values(is_current=False)
)

This UPDATE statement touched every existing revision row, causing the database to fire the ON UPDATE CURRENT_TIMESTAMP trigger on each one. As a result, all prior revisions got their updated_at overwritten to the current time — making every revision appear to have been created at the same moment.

Fix

1. Model (keep/api/models/db/workflow.py): Removed onupdate=func.now() from WorkflowVersion.updated_at. The column still gets a server_default so it is populated on INSERT, but subsequent UPDATEs no longer mutate the timestamp.

2. Migration (2025-07-01-00-00_fix_workflowversion_updated_at.py): Alters the column definition in-place (using batch_alter_table for SQLite compatibility) to drop the ON UPDATE behaviour while keeping existing data intact.

Testing

• Create a workflow and save it (revision 1).
• Wait a few seconds, then save again (revision 2).
• Open the Versions panel — revision 1 should still show its original timestamp, not the time of the revision-2 save.

Checklist

• Bug fix (non-breaking change)
• Includes a database migration
• Existing revision timestamps are preserved (migration does not touch data, only column metadata)

[DragonBot00]

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