Add notification entities, DbContext config, and migration

Creates MemberChannelBinding, LineBindingCode, MessagingGroup, and NotificationLog
entities under ROLAC.API.Entities.Notifications; wires DbSets and fluent config into
AppDbContext; generates EF migration AddNotifications creating the four tables.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

API/ROLAC.API/Entities/Notifications/LineBindingCode.cs

@@ -0,0 +1,14 @@
+using ROLAC.API.Entities;
+
+namespace ROLAC.API.Entities.Notifications;
+
+/// <summary>A short-lived code a member types to the Line bot to complete account binding.</summary>
+public class LineBindingCode
+{
+    public int       Id         { get; set; }
+    public string    Code       { get; set; } = null!;
+    public int       MemberId   { get; set; }
+    public Member?   Member     { get; set; }
+    public DateTime  ExpiresAt  { get; set; }
+    public DateTime? ConsumedAt { get; set; }            // null = unused
+}

API/ROLAC.API/Entities/Notifications/MemberChannelBinding.cs

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+using ROLAC.API.Entities;
+
+namespace ROLAC.API.Entities.Notifications;
+
+/// <summary>
+/// Binds a member to an external channel account (e.g. a Line userId). Separate table so future
+/// channels don't require changes to Member.
+/// </summary>
+public class MemberChannelBinding
+{
+    public int      Id         { get; set; }
+    public int      MemberId   { get; set; }
+    public Member?  Member     { get; set; }
+    public string   Channel    { get; set; } = null!;   // "line"
+    public string   ExternalId { get; set; } = null!;   // Line userId
+    public DateTime BoundAt    { get; set; }
+}

API/ROLAC.API/Entities/Notifications/MessagingGroup.cs

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+namespace ROLAC.API.Entities.Notifications;
+
+/// <summary>A Line group the bot was added to. Named by an admin after the join event.</summary>
+public class MessagingGroup
+{
+    public int      Id           { get; set; }
+    public string   Channel      { get; set; } = null!;  // "line"
+    public string   ExternalId   { get; set; } = null!;  // Line groupId
+    public string?  Name         { get; set; }
+    public bool     IsActive     { get; set; } = true;
+    public DateTime RegisteredAt { get; set; }
+}

API/ROLAC.API/Entities/Notifications/NotificationLog.cs

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+using ROLAC.API.Entities;
+
+namespace ROLAC.API.Entities.Notifications;
+
+/// <summary>An append-only audit row for every email or Line send (success or failure).</summary>
+public class NotificationLog
+{
+    public long     Id               { get; set; }
+    public string   Channel          { get; set; } = null!;  // "email" | "line"
+    public string   TargetType       { get; set; } = null!;  // "email" | "user" | "group"
+    public string   TargetExternalId { get; set; } = null!;  // email address OR Line id
+    public string?  Subject          { get; set; }            // email only
+    public int?     MemberId         { get; set; }
+    public Member?  Member           { get; set; }
+    public int?     MessagingGroupId { get; set; }
+    public MessagingGroup? MessagingGroup { get; set; }
+    public string   Body             { get; set; } = null!;
+    public string   Status           { get; set; } = null!;  // "sent" | "failed"
+    public string?  Error            { get; set; }
+    public string   SentByUserId     { get; set; } = null!;
+    public DateTime SentAt           { get; set; }
+}