Add collector settings and per-source purge

Every limit the fetcher was using was a constant. They are settings now, and
CollectOptions became the single place policy lives: AppSettings.ToCollectOptions
clamps them, and the HTTP layer's FetchOptions is projected from that. One
clamping site rather than two sets of ceilings drifting apart.

Clamping rather than validating, for the reason the proxy options already do it:
a hand-edited file must not stop the app from starting. A MaxItemBytes edited to
zero would otherwise refuse everything, and a zeroed concurrency would deadlock
the run outright - so both are pulled into range instead. An empty format filter
is read as "everything", because switching every format off is far more likely
to be a slip than an instruction to collect nothing.

The media root has an ordering problem - it is a setting that decides the paths
the container is built from - so the file is read once before the container
exists rather than making every path lazy for one value.

Purge is scoped to a source and lives on the Collect page, where the source is
already chosen. Content another source also holds survives, which is what the
index's reference count was for.

The showcase hint says out loud what a hard link means: editing the browsable
copy edits the original, and deleting it frees nothing until the last name goes.
That is surprising enough to belong in the UI rather than only in the code.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/AvParser.Infrastructure.Tests/JsonSettingsServiceTests.cs

@@ -1,3 +1,4 @@
+using AvParser.Core.Collecting;
 using AvParser.Core.Proxies;
 using AvParser.Core.Settings;
 using AvParser.Infrastructure.Settings;
@@ -79,6 +80,90 @@ public sealed class JsonSettingsServiceTests : IDisposable
         settings.MinimumLogLevel.ShouldBe("Information");
     }
 
+    /// <summary>
+    /// The same guarantee, for the collector settings added afterwards.
+    /// </summary>
+    /// <remarks>
+    /// The failure this prevents is worse here than it was for the proxy pool: a zeroed
+    /// <c>MaxItemBytes</c> would refuse everything, and a zeroed concurrency would deadlock the
+    /// run outright.
+    /// </remarks>
+    [Fact]
+    public void A_file_that_predates_the_collector_keeps_the_collector_defaults()
+    {
+        WriteSettings("""{ "theme": "Dark", "proxyMinimumLive": 25 }""");
+
+        using var service = Create();
+        var settings = service.Current;
+
+        settings.MaxConcurrentDownloads.ShouldBe(4);
+        settings.MaxConcurrentPerHost.ShouldBe(2);
+        settings.HostDelayMs.ShouldBe(250);
+        settings.MaxItemBytes.ShouldBe(33_554_432);
+        settings.MinItemBytes.ShouldBe(1024);
+        settings.MaxRedirects.ShouldBe(5);
+        settings.ConnectTimeoutSeconds.ShouldBe(15);
+        settings.HeaderTimeoutSeconds.ShouldBe(30);
+        settings.IdleTimeoutSeconds.ShouldBe(20);
+        settings.AllowedMediaKinds.ShouldBe(MediaKindFilter.All);
+        settings.ShowcaseMode.ShouldBe(ShowcaseMode.HardLink);
+        settings.CollectUserAgent.ShouldNotBeNullOrWhiteSpace();
+
+        // And what the file did carry survives.
+        settings.ProxyMinimumLive.ShouldBe(25);
+    }
+
+    [Fact]
+    public void Collect_options_from_an_older_file_are_usable()
+    {
+        WriteSettings("""{ "theme": "Dark" }""");
+
+        using var service = Create();
+        var options = service.Current.ToCollectOptions();
+
+        options.MaxConcurrentDownloads.ShouldBeGreaterThan(0);
+        options.MaxItemBytes.ShouldBeGreaterThan(0);
+        options.AllowedKinds.ShouldBe(MediaKindFilter.All);
+        options.IdleTimeout.ShouldBeGreaterThan(TimeSpan.Zero);
+    }
+
+    [Fact]
+    public void Collector_values_out_of_range_are_clamped_rather_than_thrown()
+    {
+        var options = new AppSettings(
+            MaxConcurrentDownloads: 0,
+            MaxConcurrentPerHost: -5,
+            MaxItemBytes: 0,
+            MaxRedirects: 9999,
+            IdleTimeoutSeconds: 0,
+            CollectUserAgent: "   "
+        ).ToCollectOptions();
+
+        options.MaxConcurrentDownloads.ShouldBe(1);
+        options.MaxConcurrentPerHost.ShouldBe(1);
+        options.MaxItemBytes.ShouldBe(1024);
+        options.MaxRedirects.ShouldBe(20);
+        options.IdleTimeout.ShouldBe(TimeSpan.FromSeconds(1));
+        options.UserAgent.ShouldBe("AvParser/0.1");
+    }
+
+    [Fact]
+    public void An_empty_media_filter_is_treated_as_everything()
+    {
+        // Switching every format off is far more likely to be an accident than an instruction to
+        // collect nothing at all.
+        new AppSettings(AllowedMediaKinds: MediaKindFilter.None)
+            .ToCollectOptions()
+            .AllowedKinds.ShouldBe(MediaKindFilter.All);
+    }
+
+    [Fact]
+    public void The_proxy_gate_setting_reaches_the_collector()
+    {
+        new AppSettings(AllowDirectConnection: false).ToCollectOptions().RequireProxy.ShouldBeTrue();
+        new AppSettings(AllowDirectConnection: true).ToCollectOptions().RequireProxy.ShouldBeFalse();
+    }
+
     [Fact]
     public void Options_built_from_an_older_file_still_consult_the_feed()
     {