Finalize FrontOffice accept/reject: role-aware, authz, signature; drop Signature.FilePath

FrontOfficeApiController.AcceptQuery/RejectQuery now resolve the actor
(provider vs client vs admin) from the query's PerformerId/ClientId and
authorize accordingly — a non-party caller is forbidden. Either party may
accept or reject. Acceptance may carry a PostIt signature, which is staged
on the query's linked estimate (Estimate.CommandId == queryId) and sets the
estimate's ProviderValidationDate / ClientValidationDate; a signature with
no linked estimate, or a signature on a reject, returns 400. Admin bypasses
authz but cannot sign.

The shared signature-staging logic (find-or-add the Signature row by
(EstimateId, Type), write Strokes/CoordinateMax/CapturedAtUtc) is extracted
into EstimateSignaturePersister, and BillingController.Sign now calls it.
The persister owns no disk I/O and no SaveChanges — the caller persists in
its own transaction.

Signature.Strokes is now the sole source of truth: drop the FilePath
property, remove the on-disk EstimateSignatureFileHelper (and its tests),
and add the DropSignatureFilePath migration. The legacy PNG prosign/clisign
flow is independent of Signature.FilePath and is unchanged.

Co-Authored-By: Claude Code <noreply@anthropic.com>

src/Yavsc.Org/Migrations/ApplicationDbContextModelSnapshot.cs

@@ -17,7 +17,7 @@ namespace Yavsc.Migrations
         {
 #pragma warning disable 612, 618
             modelBuilder
-                .HasAnnotation("ProductVersion", "10.0.9")
+                .HasAnnotation("ProductVersion", "10.0.12")
                 .HasAnnotation("Relational:MaxIdentifierLength", 63);
 
             NpgsqlModelBuilderExtensions.UseIdentityByDefaultColumns(modelBuilder);
@@ -1521,10 +1521,6 @@ namespace Yavsc.Migrations
                     b.Property<long>("EstimateId")
                         .HasColumnType("bigint");
 
-                    b.Property<string>("FilePath")
-                        .IsRequired()
-                        .HasColumnType("text");
-
                     b.Property<string>("SignerId")
                         .IsRequired()
                         .HasColumnType("text");
@@ -3959,8 +3955,8 @@ namespace Yavsc.Migrations
             modelBuilder.Entity("Yavsc.Models.BlogSpotPublication", b =>
                 {
                     b.HasOne("Yavsc.Models.Blog.BlogPost", "Post")
-                        .WithMany()
-                        .HasForeignKey("PostId")
+                        .WithOne("Publication")
+                        .HasForeignKey("Yavsc.Models.BlogSpotPublication", "PostId")
                         .OnDelete(DeleteBehavior.Cascade)
                         .IsRequired();
 
@@ -4614,6 +4610,9 @@ namespace Yavsc.Migrations
 
                     b.Navigation("Comments");
 
+                    b.Navigation("Publication")
+                        .IsRequired();
+
                     b.Navigation("Tags");
                 });