Replace chapter setting with multi-select locations; add chapter character summary

Chapters now carry many Locations (new Tags-style entity with cross-referencing)
instead of a single free-text Setting field, with a Locations tab on the novel
for browsing them and seeing every chapter set at each one. Also surfaces the
distinct characters appearing in a chapter's beats, linked, under the beat/word
count on the outline tab.

src/Novelly.Api/Chapters/Chapter.cs

@@ -2,6 +2,7 @@ using Microsoft.EntityFrameworkCore;
 using Microsoft.EntityFrameworkCore.Metadata.Builders;
 using Novelly.Api.Beats;
 using Novelly.Api.Common;
+using Novelly.Api.Locations;
 using Novelly.Api.Novels;
 using Novelly.Api.Tags;
 
@@ -19,7 +20,6 @@ public class Chapter
 
     public string? Summary { get; set; }
 
-    public string? Setting { get; set; }
     public string? Notes { get; set; }
 
     public DraftStatus Status { get; set; } = DraftStatus.Planned;
@@ -35,6 +35,7 @@ public class Chapter
     public List<Beat> Beats { get; set; } = [];
 
     public List<Tag> Tags { get; set; } = [];
+    public List<Location> Locations { get; set; } = [];
 }
 
 public class ChapterEntityTypeConfiguration : IEntityTypeConfiguration<Chapter>