Add FK from ProposalProvenance to AutomationProposal in EF config

ProposalProvenance.ProposalId was indexed but had no foreign key
constraint, allowing orphaned provenance rows. Configure a one-to-one
relationship with cascade delete to match the existing pattern used
by Operations, Revisions, and Outcomes.

Author: Chris0Jeky

backend/src/Taskdeck.Infrastructure/Persistence/Configurations/ProposalProvenanceConfiguration.cs

@@ -36,6 +36,11 @@ public void Configure(EntityTypeBuilder<ProposalProvenance> builder)
             .IsRequired()
             .IsConcurrencyToken();
 
+        builder.HasOne<AutomationProposal>()
+            .WithOne()
+            .HasForeignKey<ProposalProvenance>(pp => pp.ProposalId)
+            .OnDelete(DeleteBehavior.Cascade);
+
         builder.HasIndex(pp => pp.ProposalId)
             .IsUnique();