Optimize scheduler backfill completion updates

[fat-catTW]

Why:

The scheduler checks whether backfills are complete as part of its regular loop. The previous implementation loaded every matching Backfill row into the ORM session and then updated each object in Python, even though the completion condition can be expressed entirely in SQL.

Using a bulk update keeps this scheduler path lighter while preserving the existing AIP-78 backfill lifecycle guards.

Solution:

Update _mark_backfills_complete() to issue a single bulk UPDATE for backfills that are ready to complete.

The existing guards are preserved:

• initializing backfills without associations are skipped
• orphaned backfills older than the cutoff can still be completed
• backfills with queued or running Dag runs stay active

Add a regression test that verifies the completion path uses a single update statement.

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