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.

Makefile

@@ -5,3 +5,15 @@ test:
 .PHONY: release
 release:
 	curl -sL https://git.io/goreleaser | bash
+
+.PHONY: sync-upstream
+sync-upstream:
+	git fetch upstream
+	git checkout automc
+	git rebase upstream/main
+	go build ./...
+	go test ./internal/automc/...
+
+.PHONY: automc-build
+automc-build:
+	go build -o mc-router ./cmd/mc-router