fix: prevent auth provider cache staleness

[JustAGhosT]

Summary

• stop HTTP caching /api/auth/providers so runtime Mystira Identity config changes show immediately
• fetch auth providers with cache: no-store on login and signup pages

Live finding

The live providers endpoint currently returns Mystira Identity, but it was still serving Cache-Control: public, max-age=300. A prior empty provider response could make the login UI show the stale "Mystira Identity sign-in is not configured" hint even after the App Service config was corrected.

Validation

• pnpm exec eslint app/api/auth/providers/route.ts app/login/page.tsx app/signup/page.tsx
• pnpm exec prettier --check app/api/auth/providers/route.ts app/login/page.tsx app/signup/page.tsx
• pnpm run build
• pnpm run type-check

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Bypass the provider cache for no-store responses

When an external/OIDC provider list has already been fetched, this endpoint still calls getAvailableProviders(), which returns the process-level providersCache until its 5-minute TTL expires (lib/auth/identity-provider.ts:198, with writes at 215-218 and 262-265). In that scenario, toggling or changing providers in the identity service can still leave login/signup serving stale provider data despite the new no-store HTTP header and client fetch option; only browser/proxy caching is bypassed, not the in-memory provider cache.

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