feat(prisma): link alerts to incidents

[lishmanTech]

Summary

• Add an optional relationship between Alert and Incident.
• Add an index on alerts.incidentId for efficient incident-based alert lookups.
• Add the corresponding foreign key constraint with ON DELETE SET NULL.
• Generate the Prisma migration for the new relationship.

Implementation

Alerts can now optionally belong to an incident, allowing multiple related alerts to be grouped under a single security incident.

The relationship is nullable so existing alerts can remain unassigned to an incident.

Validation

• npx prisma format ✅
• npx prisma validate ✅
• Prisma migration generated successfully.

Closes #118