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/engine/urgency.go

@@ -170,8 +170,12 @@ func (t *Task) urgencyProject(coeff float64) float64 {
 	return 0.0
 }
 
-// BuildUrgencyCoefficients creates UrgencyCoefficients from config
+// BuildUrgencyCoefficients creates UrgencyCoefficients from config.
+// If cfg is nil, uses DefaultConfig() to prevent nil-pointer panics.
 func BuildUrgencyCoefficients(cfg *Config) *UrgencyCoefficients {
+	if cfg == nil {
+		cfg = DefaultConfig()
+	}
 	return &UrgencyCoefficients{
 		Due:         cfg.UrgencyDue,
 		PriorityH:   cfg.UrgencyPriorityH,