Add definition-level UUID generation for vulnerabilities

Summary

This MR implements Phase 1 of the Vulnerability UUID migration to support the definition model transition.

Changes

• Added generic_vuln_signature_uuid column to vulnerabilities table
• Extended Security::VulnerabilityUUID with generate_for_vulnerability method that excludes location fingerprint
• Populates generic_vuln_signature_uuid on vulnerability creation via create services

UUID Composition

The new definition-level UUID (generic_vuln_signature_uuid) includes:

• Report type
• Primary identifier fingerprint
• Project ID

NOT included (unlike Finding UUID):

• Location fingerprint
• Security tracked context ID

This allows the same vulnerability type across different locations to share the same generic_vuln_signature_uuid, making it easier to compare vulnerabilities without implementation context.

Migration Phases

Phase	Issue	Description
1	#587624 (this MR)	Start populating generic_vuln_signature_uuid for new vulnerabilities
2	#587802	Backfill generic_vuln_signature_uuid for existing vulnerabilities
3	#587803	Add NOT NULL constraint to generic_vuln_signature_uuid
4	#587804	Transition GitLab to use generic_vuln_signature_uuid
5	#587805	Drop old uuid column and rename

Closes #587624