feat: initial Jellyfin OIDC auth plugin

OpenID Connect SSO for Jellyfin 10.10 (net8.0). Authorization code flow
with PKCE via IdentityModel.OidcClient, automatic user creation with
random password, role based admin/access mapping, plugin repo manifest.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

build.yaml

@@ -0,0 +1,19 @@
+---
+name: "OIDC Auth"
+guid: "96badb44-9940-4ff0-befc-5f987159152c"
+version: "1.0.0.0"
+targetAbi: "10.10.0.0"
+framework: "net8.0"
+owner: "pascallinxweiler"
+overview: "Log in to Jellyfin with an OpenID Connect provider"
+description: >
+  Adds OpenID Connect (OIDC) single sign-on to Jellyfin. Supports any
+  spec-compliant provider (Authelia, Authentik, Keycloak, Pocket ID, Google, ...),
+  automatic user creation, and role based admin/access mapping.
+category: "Authentication"
+artifacts:
+  - "Jellyfin.Plugin.OidcAuth.dll"
+  - "IdentityModel.OidcClient.dll"
+  - "IdentityModel.dll"
+changelog: |
+  Initial release.