feat: auto-redirect to OIDC IdP (#991)

[DerDummePunkt]

This PR attempts to implement the missing part of #991

Add GOTIFY_OIDC_AUTO_REDIRECT (with default false) to skip the login page and redirect straight to the configured OIDC provider. Only takes effect when local auth is disabled, since local login would otherwise be unreachable.

Add GOTIFY_OIDC_AUTO_REDIRECT_REQUIRE_REAUTH (default false) to send prompt=login on that redirect, so logging out of Gotify doesn't silently log the user back in via an existing IdP session. Does not end that IdP session, so other apps using it are unaffected.

Wire both flags through gotifyinfo/injected UI config and the WebUI login page, which now redirects instead of showing the OIDC button when enabled.

Added helperfunction to emit warnigns via FutureLog in config builder/parser

[codecov]

Codecov Report

❌ Patch coverage is 96.55172% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 75.47%. Comparing base (8af44e0) to head (82856b6).

Files with missing lines	Patch %	Lines
api/oidc.go	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1029      +/-   ##
==========================================
+ Coverage   74.78%   75.47%   +0.69%     
==========================================
  Files          66       66              
  Lines        3541     3556      +15     
==========================================
+ Hits         2648     2684      +36     
+ Misses        688      662      -26     
- Partials      205      210       +5     

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[jmattheis]

Thanks for the contribution.

[DerDummePunkt]

Im pretty confident the prompt params are correct and to spec, but I'm having trouble testing value combinations, it seems I have discovered a bug in authelia 🤷‍♂️

I will test mutliple prompt values against one of my dev keycloak instances later, but configuring keycloak is always such a pain in the a**, I doint have time for that right now

[jmattheis]

I've made some changes, notably:

• Made the prompt a text-list
• Added the prompt parameter to the external auth flow
• The login form is always shown (no login spinner), and no redirect is done when oidc is disabled or there is a connection error

I'll test this some more this week.

[jmattheis]

I've tested with dex, authelia, keycloak and authentik. Authentik didn't seem to use the prompt parameter, but didn't error when it was set. I tried using multiple prompt values like login consent, but this didn't seem to make any difference on the listed IdPs. authelia prompt for consent anyway when using prompt=login.