Implement core backend domain and API workflows

RpgRoller/Domain/GameModels.cs

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+namespace RpgRoller.Domain;
+
+public enum RulesetKind
+{
+    D6,
+    Dnd5e
+}
+
+public enum RollVisibility
+{
+    Public,
+    Private
+}
+
+public sealed class UserAccount
+{
+    public required Guid Id { get; init; }
+    public required string Username { get; init; }
+    public required string PasswordHash { get; set; }
+    public required string DisplayName { get; set; }
+}
+
+public sealed class UserSession
+{
+    public required string Token { get; init; }
+    public required Guid UserId { get; init; }
+    public required DateTimeOffset CreatedAtUtc { get; init; }
+}
+
+public sealed class Campaign
+{
+    public required Guid Id { get; init; }
+    public required Guid GmUserId { get; init; }
+    public required string Name { get; set; }
+    public required RulesetKind Ruleset { get; set; }
+    public long Version { get; set; }
+}
+
+public sealed class Character
+{
+    public required Guid Id { get; init; }
+    public required Guid OwnerUserId { get; init; }
+    public required Guid CampaignId { get; set; }
+    public required string Name { get; set; }
+}
+
+public sealed class Skill
+{
+    public required Guid Id { get; init; }
+    public required Guid CharacterId { get; set; }
+    public required string Name { get; set; }
+    public required string DiceRollDefinition { get; set; }
+}
+
+public sealed class RollLogEntry
+{
+    public required Guid Id { get; init; }
+    public required Guid CampaignId { get; init; }
+    public required Guid CharacterId { get; init; }
+    public required Guid SkillId { get; init; }
+    public required Guid RollerUserId { get; init; }
+    public required RollVisibility Visibility { get; init; }
+    public required int Result { get; init; }
+    public required string Breakdown { get; init; }
+    public required DateTimeOffset TimestampUtc { get; init; }
+}
+
+public sealed record DiceExpression(int DiceCount, int Sides, int Modifier, string Canonical);