Add load balancer with round-robin, failover, and weighted strategies

Implements balancer middleware with URL rewriting per-request:
- RoundRobin, Failover, and WeightedRandom endpoint selection strategies
- Background HealthChecker with configurable probe interval and path
- Thread-safe health state tracking with sync.RWMutex

balancer/failover.go

@@ -0,0 +1,17 @@
+package balancer
+
+type failover struct{}
+
+// Failover returns a strategy that always picks the first healthy endpoint.
+// If the primary endpoint is unhealthy, it falls back to the next available
+// healthy endpoint in order.
+func Failover() Strategy {
+	return &failover{}
+}
+
+func (f *failover) Next(healthy []Endpoint) (Endpoint, error) {
+	if len(healthy) == 0 {
+		return Endpoint{}, ErrNoHealthy
+	}
+	return healthy[0], nil
+}