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/Common/RequestLoggingEndpointFilter.cs

@@ -0,0 +1,24 @@
+namespace Novelly.Api.Common;
+
+/// <summary>
+/// Logs every request an endpoint group handles: Information on entry with the route's
+/// name and values, Debug on exit with the resulting status. Applied per <c>MapGroup</c>
+/// rather than inside each handler, so no endpoint lambda needs to know about logging.
+/// </summary>
+public class RequestLoggingEndpointFilter(ILogger<RequestLoggingEndpointFilter> logger) : IEndpointFilter
+{
+    public async ValueTask<object?> InvokeAsync(EndpointFilterInvocationContext context, EndpointFilterDelegate next)
+    {
+        var routeName = context.HttpContext.GetEndpoint()?.DisplayName ?? context.HttpContext.Request.Path;
+
+        logger.LogInformation(
+            "HTTP {Method} {Route} invoked with {@RouteValues}",
+            context.HttpContext.Request.Method, routeName, context.HttpContext.Request.RouteValues);
+
+        var result = await next(context);
+
+        logger.LogDebug("HTTP {Method} {Route} completed with {ResultType}", context.HttpContext.Request.Method, routeName, result?.GetType().Name);
+
+        return result;
+    }
+}