fix: prevent nil-panic on server and improve OAuth callback handling

Load config eagerly during server startup so sortByUrgency never
hits a nil config. Add nil-guard in BuildUrgencyCoefficients as
belt-and-suspenders defense. Fix OAuth callback to support both
GET and POST, and resolve issuer URLs properly with path.Dir.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

opal-task/internal/api/server.go

@@ -41,6 +41,7 @@ func (s *Server) setupRoutes() {
 
 	// OAuth endpoints (no auth required)
 	r.Get("/auth/login", handlers.GetLoginURL)
+	r.Get("/auth/callback", handlers.OAuthCallback)
 	r.Post("/auth/callback", handlers.OAuthCallback)
 	r.Post("/auth/refresh", handlers.RefreshToken)
 	r.Post("/auth/logout", handlers.Logout)