Add Serilog console logging across the API

Information at endpoint and service-method boundaries, Debug in deeper
helpers, Warning before expected/recoverable failures (not-found,
validation, agent tool errors), Error on caught exceptions. Serilog
wraps the exception handler so request-completion logs report the
resolved status code rather than the raw exception. Never logs prose
bodies or the Anthropic API key.

src/Novelly.Api/appsettings.json

@@ -6,6 +6,17 @@
       "Microsoft.EntityFrameworkCore.Database.Command": "Warning"
     }
   },
+  "Serilog": {
+    "MinimumLevel": {
+      "Default": "Information",
+      "Override": {
+        "Novelly": "Information",
+        "Microsoft.AspNetCore": "Warning",
+        "Microsoft.AspNetCore.Diagnostics.ExceptionHandlerMiddleware": "Fatal",
+        "Microsoft.EntityFrameworkCore.Database.Command": "Warning"
+      }
+    }
+  },
   "AllowedHosts": "*",
   "ConnectionStrings": {
     "Novel": "Data Source=novel.db"