feat: add OAuth debug route and template for URL generation verification

.env.oauth

@@ -0,0 +1,18 @@
+# Standard OAuth redirect URIs
+SPOTIFY_REDIRECT_URI=http://localhost:5000/auth/spotify/callback
+GOOGLE_REDIRECT_URI=http://localhost:5000/users/login/google/callback
+AUTHENTIK_REDIRECT_URI=http://localhost:5000/users/login/authentik/callback
+DROPBOX_REDIRECT_URI=http://localhost:5000/users/dropbox/callback
+
+# OAuth settings
+USE_HTTPS=False                   # Set to True when behind a reverse proxy with HTTPS offloading
+# PREFERRED_URL_SCHEME=https      # Optional, set automatically if USE_HTTPS is True
+
+# Static OAuth redirect URIs for production environments
+# These override the dynamic URLs generated by url_for() when specified
+# STATIC_OAUTH_URLS=True          # Uncomment to use static URLs below
+# OAUTH_SPOTIFY_AUTH_URL=https://your-domain.com/auth/spotify/callback
+# OAUTH_SPOTIFY_LINK_URL=https://your-domain.com/users/spotify-link/callback
+# OAUTH_GOOGLE_URL=https://your-domain.com/users/login/google/callback
+# OAUTH_AUTHENTIK_URL=https://your-domain.com/users/login/authentik/callback
+# OAUTH_DROPBOX_URL=https://your-domain.com/users/dropbox/callback