signup: close two concurrency holes from final review

(1) RegisterAnonymousViewer now catches the unique-violation
    race (SQLSTATE 23505 on Postgres / code 19 on SQLite) and
    re-reads by UDID, returning the existing row instead of
    surfacing 500 to the second concurrent /tool/signup caller.
    New repo test exercises the back-to-back register path.

(2) Add unique index on SocialAccountConnection (AccountType,
    AccountId). The auth handler's find-or-link path claimed
    this index existed as the dedup backstop; the claim was
    accurate as design intent but the schema was missing. Now
    matched. Comment in handler updated.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.Database/Migrations/SVSimDbContextModelSnapshot.cs

@@ -2923,6 +2923,9 @@ namespace SVSim.Database.Migrations
 
                             b1.HasKey("ViewerId", "Id");
 
+                            b1.HasIndex("AccountType", "AccountId")
+                                .IsUnique();
+
                             b1.ToTable("SocialAccountConnection");
 
                             b1.WithOwner("Viewer")