Add detected_at column to vulnerability_occurrences table

What does this MR do and why?

This MR implements the database migration requested in issue #556251 (closed) to add the detected_at column to the vulnerability_occurrences table.

With the vulnerability_occurrences table acting as the primary record representing a vulnerability as it exists within a branch context, certain columns don't make sense in their place on the higher level vulnerabilities table and need to be moved down.

This migration adds:

• detected_at timestamp with time zone DEFAULT now()

Database changes

• Adds detected_at column to vulnerability_occurrences table
• Uses timestamptz type with now() default
• Follows GitLab migration patterns for large tables with disable_ddl_transaction! and with_lock_retries

How to set up and validate locally

1. Run the migration: bundle exec rails db:migrate
2. Verify the column exists: bundle exec rails db:console and check the table schema
3. Test rollback: bundle exec rails db:rollback

MR acceptance checklist

• Database migration follows GitLab patterns
• Migration is reversible
• Uses appropriate column type and default value
• Includes proper rubocop disables for oversized table

Closes #556251 (closed)