Switch remote deploy to vendored source builds

Move remote deployment to a vendored source bundle built on the target host via Docker so redeploys no longer require local cross-compilation or host Go installation.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

vendor/github.com/valyala/fasttemplate/unsafe_gae.go

@@ -0,0 +1,11 @@
+// +build appengine
+
+package fasttemplate
+
+func unsafeBytes2String(b []byte) string {
+	return string(b)
+}
+
+func unsafeString2Bytes(s string) []byte {
+	return []byte(s)
+}