Floatplane channel plugin for Jellyfin 10.10.x: browse, HLS proxy, cookie session, CI release

- IChannel: creators -> sub-channels -> video posts (public listing API, search mapped)
- IRequiresMediaInfoCallback -> signed proxy URLs; stream controller rewrites HLS so the
  AES-128 key is fetched server-side with the session cookie, segments stay on the CDN
- delivery/info cached per token lifetime and throttled (~2/min)
- config page: paste sails.sid + validate, extra creators, quality, search test
- scripts/package.sh -> plugin folder + zip + md5 + manifest.json; Gitea release workflow
- research docs, HAR analysis (HAR itself gitignored), install-pi5.sh

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Jellyfin.Plugin.Floatplane/Api/FloatplaneExceptions.cs

@@ -0,0 +1,41 @@
+using System;
+using System.Net;
+
+namespace Jellyfin.Plugin.Floatplane.Api;
+
+/// <summary>Floatplane answered with an error envelope.</summary>
+public class FloatplaneApiException : Exception
+{
+    public FloatplaneApiException(HttpStatusCode status, string? errorName, string message)
+        : base(message)
+    {
+        Status = status;
+        ErrorName = errorName;
+    }
+
+    public HttpStatusCode Status { get; }
+
+    /// <summary>e.g. notLoggedInError, notSubscribedError, paramValidationError.</summary>
+    public string? ErrorName { get; }
+}
+
+/// <summary>The session cookie is missing, expired or not subscribed to the item.</summary>
+public class FloatplaneAuthException : FloatplaneApiException
+{
+    public FloatplaneAuthException(HttpStatusCode status, string? errorName, string message)
+        : base(status, errorName, message)
+    {
+    }
+}
+
+/// <summary>HTTP 429 from Floatplane or Cloudflare.</summary>
+public class FloatplaneRateLimitException : FloatplaneApiException
+{
+    public FloatplaneRateLimitException(TimeSpan retryAfter, string message)
+        : base(HttpStatusCode.TooManyRequests, "rateLimited", message)
+    {
+        RetryAfter = retryAfter;
+    }
+
+    public TimeSpan RetryAfter { get; }
+}