Add anonymous playback with an optional Spotify-style preview cap

New Playback:AllowAnonymousListening / Playback:AnonymousFragmentSeconds
settings control whether a request without a session can stream at all,
and if so, whether it's limited to a fragment instead of the full track:

- GET /tracks/{id}/stream and GET /youtube/tracks/{videoId}/stream no
  longer require authorization outright; the handlers gate on the config
  instead, so both keep working exactly as before when it's off (the
  default).
- The fragment cap is enforced by the Streaming Gateway, not the client:
  the stream ticket now carries an optional maxBytes claim (computed from
  AnonymousFragmentSeconds and the transcoder's known bitrate), and
  TicketValidationMiddleware clamps the Range it forwards to SeaweedFS to
  stay under it — synthesizing one if the request didn't send one, and
  answering 416 outright if the requested range starts past the cap.
- A YouTube track that hasn't been provisioned server-side yet streams
  straight from YouTube's CDN, bypassing the Gateway entirely — there's no
  way to cap that, so an anonymous request for one is refused (409) instead
  of silently handing out an uncapped stream.
- ListeningHistory has a non-nullable UserId, so anonymous plays simply
  don't get recorded rather than trying to force a row through the FK.

Also fixes a real, pre-existing bug this surfaced: the AppSettings.Development.json
of the Api, Worker, and StreamingGateway all pointed StreamTicket/Workers
paths at "../../deploy/..." — one directory short of deploy/'s actual
location at the repo root relative to each project's own folder. Docker
never hit this (it mounts an absolute /keys path), so it had apparently
never been exercised via `dotnet run` before.

backend/Core/Musify.Application/Configuration/PlaybackConfiguration.cs

@@ -0,0 +1,30 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Musify.Application.Configuration
+{
+    /// <summary>Controls whether listeners without a session can stream music at all, and if so,
+    /// how much of a track they get to hear before being cut off (a Spotify-style preview).</summary>
+    public sealed class PlaybackConfiguration
+    {
+        public const string SectionName = "Playback";
+
+        /// <summary>When false (the default), every stream endpoint requires a signed-in user, exactly
+        /// like before this setting existed. When true, an anonymous request is issued a stream ticket
+        /// too, capped by <see cref="AnonymousFragmentSeconds"/> if that is greater than zero.</summary>
+        public bool AllowAnonymousListening { get; set; }
+
+        /// <summary>How many seconds of a track an anonymous listener may hear, enforced by the
+        /// Streaming Gateway clamping the byte range it will serve for that ticket. Zero (the default)
+        /// means no cap: anonymous listeners hear the full track, same as a signed-in user. Ignored
+        /// when <see cref="AllowAnonymousListening"/> is false.</summary>
+        [Range(0, 3600)]
+        public int AnonymousFragmentSeconds { get; set; }
+
+        /// <summary>Bytes per second of the transcoded audio, used to turn <see cref="AnonymousFragmentSeconds"/>
+        /// into a byte cap for the stream ticket. Defaults to the app's own transcoder output (128 kbps
+        /// AAC, <c>AudioTranscoder:Ffmpeg:AudioBitrate</c>) — update this alongside that setting if it
+        /// ever changes, since the two aren't wired together.</summary>
+        [Range(1, int.MaxValue)]
+        public int EstimatedAudioBytesPerSecond { get; set; } = 16_000;
+    }
+}