feat(deploy): add webhook receiver and launchd service for push-to-deploy

- cmd/webhook/main.go: HMAC-validated webhook receiver on :9797
- launchd plist: keeps webhook running, PATH includes docker
- Makefile: 'make webhook' target
- Registered as Gitea webhook id 15 on dtoro/oikos

Fixes: auto-deploy was not wired on mac-mini after the consolidation

Makefile

@@ -1,4 +1,4 @@
-.PHONY: build test test-db lint generate generate-check dev migrate seed export clean tidy
+.PHONY: build webhook test test-db lint generate generate-check dev migrate seed export clean tidy
 
 BINARY := oikos
 GO ?= go
@@ -6,6 +6,9 @@ GO ?= go
 build:
 	$(GO) build -o $(BINARY) -tags timetzdata ./cmd/oikos
 
+webhook:
+	$(GO) build -o webhook -tags timetzdata ./cmd/webhook
+
 test:
 	$(GO) test -race -cover ./...