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/Models/Viewer.cs

@@ -83,6 +83,8 @@ public class Viewer : BaseEntity<long>
 
     public List<ViewerPackOpenCount> PackOpenCounts { get; set; } = new List<ViewerPackOpenCount>();
 
+    public List<ViewerPackStarterClass> PackStarterClasses { get; set; } = new List<ViewerPackStarterClass>();
+
     public List<ViewerFreePackClaim> FreePackClaims { get; set; } = new List<ViewerFreePackClaim>();
 
     public List<MyPageBgRotationEntry> MyPageBgRotation { get; set; } = new List<MyPageBgRotationEntry>();

SVSim.Database/Models/ViewerPackStarterClass.cs

@@ -0,0 +1,17 @@
+using Microsoft.EntityFrameworkCore;
+
+namespace SVSim.Database.Models;
+
+/// <summary>
+/// Records a viewer's class choice for a <see cref="Enums.PackCategory.RotationStarterCardPack"/>.
+/// One row per (Viewer, Pack) — the choice is one-shot per pack per
+/// /pack/set_rotation_starter_class spec. Surfaces as <c>selected_class_id</c> on the parent
+/// PackConfig in the next /pack/info response so the client's starter-pack dialog can
+/// pre-select on revisit. <see cref="PackId"/> = parent_gacha_id; <see cref="ClassId"/> is 1..8.
+/// </summary>
+[Owned]
+public class ViewerPackStarterClass
+{
+    public int PackId { get; set; }
+    public int ClassId { get; set; }
+}