fix(auth): restore BetterAuth OIDC flow across api/web/compose

.env.example

@@ -70,9 +70,9 @@ OIDC_ISSUER=https://auth.example.com/application/o/mosaic-stack/
 OIDC_CLIENT_ID=your-client-id-here
 OIDC_CLIENT_SECRET=your-client-secret-here
 # Redirect URI must match what's configured in Authentik
-# Development: http://localhost:3001/auth/callback/authentik
-# Production: https://api.mosaicstack.dev/auth/callback/authentik
-OIDC_REDIRECT_URI=http://localhost:3001/auth/callback/authentik
+# Development: http://localhost:3001/auth/oauth2/callback/authentik
+# Production: https://api.mosaicstack.dev/auth/oauth2/callback/authentik
+OIDC_REDIRECT_URI=http://localhost:3001/auth/oauth2/callback/authentik
 
 # Authentik PostgreSQL Database
 AUTHENTIK_POSTGRES_USER=authentik
@@ -116,6 +116,9 @@ JWT_EXPIRATION=24h
 # This is used by BetterAuth for session management and CSRF protection
 # Example: openssl rand -base64 32
 BETTER_AUTH_SECRET=REPLACE_WITH_RANDOM_SECRET_MINIMUM_32_CHARS
+# Optional explicit BetterAuth origin for callback/error URL generation.
+# When empty, backend falls back to NEXT_PUBLIC_API_URL.
+BETTER_AUTH_URL=
 
 # Trusted Origins (comma-separated list of additional trusted origins for CORS and auth)
 # These are added to NEXT_PUBLIC_APP_URL and NEXT_PUBLIC_API_URL automatically