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/Tracks/GetTrackStream.cs

@@ -2,23 +2,30 @@ using ErrorOr;
 using Mediator;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Logging;
+using Musify.Application.Configuration;
 using Musify.Application.Contracts;
 using Musify.Application.Services;
 using Musify.Application.Tracks.Responses;
 
 namespace Musify.Application.Tracks
 {
-    public record GetTrackStreamQuery(Guid TrackId, long UserId)
+    /// <param name="UserId">Null for an anonymous request — allowed only when
+    /// <see cref="PlaybackConfiguration.AllowAnonymousListening"/> is enabled.</param>
+    public record GetTrackStreamQuery(Guid TrackId, long? UserId)
         : IQuery<ErrorOr<TrackStreamResponse>>;
 
     public class GetTrackStreamQueryHandler(
         IDatabase database,
         TrackStreamIssuer streamIssuer,
+        PlaybackConfiguration playbackConfiguration,
         ILogger<GetTrackStreamQueryHandler> logger)
         : IQueryHandler<GetTrackStreamQuery, ErrorOr<TrackStreamResponse>>
     {
         public async ValueTask<ErrorOr<TrackStreamResponse>> Handle(GetTrackStreamQuery request, CancellationToken cancellationToken)
         {
+            if (request.UserId is null && !playbackConfiguration.AllowAnonymousListening)
+                return Error.Unauthorized(description: "Sign in to stream music, or ask an administrator to enable anonymous listening.");
+
             var track = await database.Tracks.AsNoTracking()
                 .Where(t => t.Id == request.TrackId)
                 .Select(t => new { t.Id, t.Audio })
@@ -35,7 +42,7 @@ namespace Musify.Application.Tracks
 
             var response = await streamIssuer.IssueAsync(track.Id, track.Audio.FolderName, request.UserId, cancellationToken);
 
-            logger.LogInformation("Issued stream ticket for track {TrackId} to user {UserId}", request.TrackId, request.UserId);
+            logger.LogInformation("Issued stream ticket for track {TrackId} to user {UserId}", request.TrackId, request.UserId?.ToString() ?? "(anonymous)");
 
             return response;
         }