[FEAT] Option to enforce OIDC SSO and disable local login

[jonhnjacobjingle]

Feature Category

• Playback / Streaming
• UI / Interface
• Source Management (Xtream/M3U)
• EPG / Program Guide
• Settings / Configuration
• Other

Is your feature request related to a problem?
The local username/password login form is still shown when no local users exist (e.g., when OIDC is active and user accounts were created through it).

Describe the feature you'd like
Add env variables to disable local login and enforce OIDC:

• DISABLE_LOCAL_AUTH=true → completely disable local login and remove option from /login.html.
• OIDC_AUTO_REDIRECT=true → automatically redirect users landing at /login.html to OIDC flow.

Use Case

• Enforce SSO-only access.
• Reduce user confusion by removing the unused local login.