harden(gateway): prefix-boundary + path-traversal checks; simplify ticket validation

Tighten the stream-ticket authorization in the gateway middleware:
- Enforce a path-segment boundary on the prefix check instead of a bare
  StartsWith, so a ticket for 'users/12' no longer grants 'users/1234/...'
  (exact-match still allowed for single-object grants)
- Reject object keys containing '..'/'.' segments before proxying, closing a
  path-traversal bypass that SeaweedFS would otherwise resolve outside the
  authorized prefix

Also simplify TicketValidator.ValidateTicketAsync from ErrorOr<string> to
string? (null = invalid) since it has a single failure mode; drop the now
unused ErrorOr package reference from the gateway.

Verified: build clean; the authorization predicate passes a table of cases
covering both the escalation and traversal vulnerabilities.

Build: 0 warnings, 0 errors.

backend/Musify.StreamingGateway/Middleware/TicketValidationMiddleware.cs

@@ -28,19 +28,27 @@ public sealed class TicketValidationMiddleware(
             token = context.Request.Headers[options.HeaderName].ToString();
         }
 
-        var validation = await ticketValidator.ValidateTicketAsync(token);
+        var prefix = await ticketValidator.ValidateTicketAsync(token);
 
-        if (validation.IsError)
+        if (prefix is null)
         {
             logger.LogWarning("Rejected media request {Path}: missing or invalid ticket", path);
             context.Response.StatusCode = StatusCodes.Status401Unauthorized;
             return;
         }
 
-        var prefix = validation.Value;
-
         var objectKey = remaining.Value?.TrimStart('/') ?? string.Empty;
-        if (!objectKey.StartsWith(prefix, StringComparison.Ordinal))
+
+        if (HasTraversalSegment(objectKey))
+        {
+            logger.LogWarning("Rejected media request {ObjectKey}: contains a path traversal segment", objectKey);
+            context.Response.StatusCode = StatusCodes.Status400BadRequest;
+            return;
+        }
+
+        var prefixBoundary = prefix.EndsWith('/') ? prefix : prefix + "/";
+        if (!string.Equals(objectKey, prefix, StringComparison.Ordinal)
+            && !objectKey.StartsWith(prefixBoundary, StringComparison.Ordinal))
         {
             logger.LogWarning("Rejected media request {ObjectKey}: outside authorized prefix {Prefix}", objectKey, prefix);
             context.Response.StatusCode = StatusCodes.Status403Forbidden;
@@ -51,6 +59,9 @@ public sealed class TicketValidationMiddleware(
         await next(context);
     }
 
+    private static bool HasTraversalSegment(string objectKey) =>
+        objectKey.Split('/').Any(segment => segment is ".." or ".");
+
     private static void StripQueryParameter(HttpContext context, string name)
     {
         var remaining = context.Request.Query