feat(pack): /pack/set_rotation_starter_class + selected_class_id read-back

Implements Task 9 from docs/superpowers/plans/2026-06-13-decomp-only-followups.md
— the pre-purchase commit endpoint the client fires from StarterClassSelectDialog
before opening a RotationStarterCardPack. Without this the per-class draw path
from yesterday's work is unreachable: the dialog blocks on the AJAX call and
never advances to /pack/open.

Wire surface:
- POST /pack/set_rotation_starter_class { pack_id, class_id } → EmptyData.
  Validates 1..8, rejects unknown packs (404), non-RS packs (400), and second
  commits (400 — one-shot per pack per spec).
- PackConfigDto carries selected_class_id (nullable, omitted via global
  WhenWritingNull policy when unset). Placement verified against decomp at
  PackInfoTask.cs:86 — it's on the parent PackConfig, NOT the child gacha as
  the original plan text had it.
- /pack/open cross-checks request.class_id against the persisted choice for
  RS packs; rejects 400 on missing-commit or class-mismatch so a tampered
  request can't swap pools after the choice is locked.

Storage:
- ViewerPackStarterClass owned entity; (ViewerId, PackId) unique index via the
  pattern from project_owned_collection_unique_index memory.
- Migration PackStarterClass — composite PK + FK cascade verified against
  fresh Postgres bootstrap.

Tests (9 new in PackControllerSetRotationStarterClassTests):
- Round-trip set → /pack/info shows selected_class_id.
- Field absent before any commit.
- Invalid class (0/9/-1/100) → 400.
- Already-chosen rejection.
- Non-RS pack / unknown pack rejection.
- /pack/open rejects no-prior-commit AND class-mismatch; succeeds when class
  matches the persisted choice.

Full suite: 1552 passed (1543 + 9).

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

SVSim.Database/SVSimDbContext.cs

@@ -184,6 +184,13 @@ public class SVSimDbContext : DbContext
             e.HasIndex(x => new { x.PackId, x.Slot, x.Tier });
         });
         modelBuilder.Entity<Viewer>().OwnsMany(v => v.PackOpenCounts);
+        modelBuilder.Entity<Viewer>().OwnsMany(v => v.PackStarterClasses, b =>
+        {
+            // One choice per (viewer, pack) — /pack/set_rotation_starter_class is one-shot
+            // per pack. Mirrors the (ViewerId, PackId) unique-index pattern used by
+            // GachaPointBalances above (project_owned_collection_unique_index memory).
+            b.HasIndex("ViewerId", nameof(ViewerPackStarterClass.PackId)).IsUnique();
+        });
         modelBuilder.Entity<Viewer>().OwnsMany(v => v.FreePackClaims, b =>
         {
             b.WithOwner().HasForeignKey("ViewerId");