Finding UUID Migration Phase 3: Add NOT NULL constraint to new_uuid

What does this MR do and why?

Implements Phase 3 of the Finding UUID migration (#587799).

Adds a NOT NULL constraint to the new_uuid column on vulnerability_occurrences after the Phase 2 backfill completes.

Changes

Uses the 3-step approach for large tables:

1. Add constraint without validation (20260127164510)

   • Adds the constraint but doesn't validate existing rows (fast)

2. Prepare async validation (20260127164526)

   • Schedules background validation of existing rows

3. Validate constraint (20260127164542)

   • Final validation (fast because async already validated)

Dependencies

This MR targets the Phase 2 branch and cannot be merged until:

• Phase 1 (!220835 (merged)) merges
• Phase 2 (!220866) merges

Migration Plan

Phase	Issue	MR	Status
1	#587625 (closed)	!220835 (merged)	In Review
2	#587798	!220866	Pipeline Running
3	#587799	This MR	Ready for Review
4	#587800	-	Not Started
5	#587801	-	Not Started

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist.

Closes #587799