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>

manifest.json

@@ -0,0 +1,20 @@
+[
+    {
+        "guid": "96badb44-9940-4ff0-befc-5f987159152c",
+        "name": "OIDC Auth",
+        "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.",
+        "overview": "Log in to Jellyfin with an OpenID Connect provider",
+        "owner": "YOUR_GITHUB_USER",
+        "category": "Authentication",
+        "versions": [
+            {
+                "version": "1.0.0.0",
+                "changelog": "Initial release.",
+                "targetAbi": "10.10.0.0",
+                "sourceUrl": "https://github.com/YOUR_GITHUB_USER/jellyfinoidc/releases/download/v1.0.0.0/oidc-auth_1.0.0.0.zip",
+                "checksum": "178db5af78c0a5d7cb0d1910338f5fde",
+                "timestamp": "2026-07-13T00:00:00Z"
+            }
+        ]
+    }
+]