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.

go.mod

@@ -8,6 +8,7 @@ require (
 	github.com/gorilla/mux v1.8.1
 	github.com/influxdata/influxdb1-client v0.0.0-20220302092344-a9ab5670611c
 	github.com/itzg/go-flagsfiller v1.17.0
+	github.com/jackc/pgx/v5 v5.8.0
 	github.com/juju/ratelimit v1.0.2
 	github.com/pires/go-proxyproto v0.11.0
 	github.com/pkg/errors v0.9.1
@@ -30,6 +31,9 @@ exclude google.golang.org/genproto v0.0.0-20210917145530-b395a37504d4
 
 require (
 	github.com/beorn7/perks v1.0.1 // indirect
+	github.com/jackc/pgpassfile v1.0.0 // indirect
+	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
+	github.com/jackc/puddle/v2 v2.2.2 // indirect
 	github.com/cespare/xxhash/v2 v2.3.0 // indirect
 	github.com/containerd/errdefs v1.0.0 // indirect
 	github.com/containerd/errdefs/pkg v0.3.0 // indirect