automc: pg LISTEN/NOTIFY route source + HTTP waker

Adds opt-in extension package internal/automc/ that:
- Subscribes to Postgres notifications on a 'servers' table and pushes
  route changes into server.Routes (no file I/O, no fsnotify).
- Provides a WakerFunc that POSTs to a configurable HTTP control plane
  (server-manager) and polls until state=running.

When AUTOMC_DSN is unset, Wire() is a no-op and the binary behaves
exactly like upstream itzg/mc-router. Single patch site in main.go
(import + 4-line call) keeps upstream rebases trivial.

See docs/AUTOMC.md for env vars and the expected DB schema/trigger.

cmd/mc-router/main.go

@@ -9,6 +9,7 @@ import (
 	"syscall"
 
 	"github.com/itzg/go-flagsfiller"
+	"github.com/itzg/mc-router/internal/automc"
 	"github.com/itzg/mc-router/server"
 	"github.com/sirupsen/logrus"
 )
@@ -65,6 +66,10 @@ func main() {
 		logrus.WithError(err).Fatal("Could not setup server")
 	}
 
+	if err := automc.Wire(ctx); err != nil {
+		logrus.WithError(err).Fatal("automc Wire failed")
+	}
+
 	var wg sync.WaitGroup
 	wg.Go(s.Run)