ci: Validate and explicitly export SUPABASE_DB_URL #349

Job	Run time
Security & Code Quality	1m 28s
Test Suite	42s
Database Tests	1m 10s
Build & Performance	4m 36s
Enhanced Security Testing	5m 6s
Deploy to Production	0s
Rollback on Failure	0s
Deploy to Staging	0s
Performance Monitoring	0s
Performance Monitoring (Staging)	0s
	13m 2s

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