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.

tests/Novelly.Api.Tests/ListingTests.cs

@@ -85,7 +85,7 @@ public class ListingTests : ServiceTestFixture
         var agent = new NovelAgentService(
             Db.Context,
             new ScriptedModelClient([[new AgentTextBlock("Reply.")]]),
-            new NovelAgentToolset(Novels, Characters, Arcs, Chapters, Beats, Tags, Questions, NullLogger<NovelAgentToolset>.Instance),
+            new NovelAgentToolset(Novels, Characters, Arcs, Chapters, Beats, Tags, Locations, Questions, NullLogger<NovelAgentToolset>.Instance),
             Options.Create(new AgentOptions()),
             NullLogger<NovelAgentService>.Instance,
             new SendAgentMessageRequestValidator());