t5de-catalog: store and serve community product files

A Go server that takes product uploads with hub upload tokens, checks them by
t5de-api's pack rules, stores files by sha256, registers revisions with the hub
(POST /api/v1/catalog/revisions), and serves files with immutable caching;
files used only by adult revisions need an adult access token. Contract:
t5de-hub's docs/products.md.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

internal/buildinfo/buildinfo.go

@@ -0,0 +1,5 @@
+// Package buildinfo holds what release builds set with -ldflags "-X ...".
+package buildinfo
+
+// Version is the release's version, "X.Y.Z" or "X.Y.Z-nightly.N+commit".
+var Version = "dev"