Implement Watched Together shared viewing accounts

Replaces the plugin template with a working plugin that lets several
users share one viewing account while keeping their individual watched
lists accurate.

Three pieces:

- Auto-creating groups. Logging in as "alice+bob" with any named
  member's own password provisions the shared account and signs you in.
  Verified against 10.11.5: AuthenticateUser offers unmatched usernames
  to every enabled provider and re-queries afterwards, which is the hook
  this relies on. Gated on a real member password so knowing two
  usernames is not enough to create an account.

- Multi-password authentication. IRequiresResolvedUser hands us the
  resolved shared account; each member's live stored hash is checked via
  ICryptoProvider.Verify. Deliberately avoids re-entering
  UserManager.AuthenticateUser, which would trip every member's
  failed-attempt counter whenever a different member's password matched.

- One-way played-state sync. Shared account to members only, filtered to
  PlaybackFinished/TogglePlayed/Import so playback progress ticks are
  ignored. No loop guard needed: member writes carry a non-shared id.

Membership is stored as user IDs rather than re-parsed from the username,
so shared accounts can be renamed freely. The +/name collision resolves
itself because Jellyfin only consults the plugin when no local user
matches the typed name.

Targets Jellyfin 10.11.x / net9.0. Adds Gitea CI (test, build, release),
a builder image, and 34 tests covering the auth and sync rules.

Jellyfin.Plugin.WatchedTogether/Jellyfin.Plugin.WatchedTogether.csproj

@@ -0,0 +1,33 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net9.0</TargetFramework>
+    <RootNamespace>Jellyfin.Plugin.WatchedTogether</RootNamespace>
+    <GenerateDocumentationFile>true</GenerateDocumentationFile>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+    <Nullable>enable</Nullable>
+    <AnalysisMode>AllEnabledByDefault</AnalysisMode>
+    <CodeAnalysisRuleSet>../jellyfin.ruleset</CodeAnalysisRuleSet>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Jellyfin.Controller" Version="10.11.5">
+      <ExcludeAssets>runtime</ExcludeAssets>
+    </PackageReference>
+    <PackageReference Include="Jellyfin.Model" Version="10.11.5">
+      <ExcludeAssets>runtime</ExcludeAssets>
+    </PackageReference>
+  </ItemGroup>
+
+  <ItemGroup>
+    <PackageReference Include="SerilogAnalyzer" Version="0.15.0" PrivateAssets="All" />
+    <PackageReference Include="StyleCop.Analyzers" Version="1.2.0-beta.556" PrivateAssets="All" />
+    <PackageReference Include="SmartAnalyzers.MultithreadingAnalyzer" Version="1.1.31" PrivateAssets="All" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <None Remove="Configuration\configPage.html" />
+    <EmbeddedResource Include="Configuration\configPage.html" />
+  </ItemGroup>
+
+</Project>