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.

manifest.json

@@ -0,0 +1,11 @@
+[
+  {
+    "guid": "aa3288a0-e8c1-43e2-8045-8c3411142a5b",
+    "name": "Watched Together",
+    "description": "Lets several users share a single viewing account. Any member's password unlocks the shared account, and anything marked watched or unwatched there propagates one-way to each member's individual account. This is not synchronized playback - for that, use Jellyfin's built-in SyncPlay.",
+    "overview": "One shared login for several people; watched state flows back to each member's own account",
+    "owner": "dtourolle",
+    "category": "General",
+    "versions": []
+  }
+]