docs(guide,architecture): move internal env vars out of the operator guide

Internal (non-operator) env vars were listed in guide.md's Environment
variables section even though the guide's own text says operators
aren't meant to touch them. They now live in architecture.md §
Configuration, the as-built/internals doc; guide.md keeps a one-line
pointer.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

cmd/panel/envdoc_test.go

@@ -17,7 +17,7 @@ var documentedPublic = []string{
 	"TRUSTED_PROXY_CIDR",
 }
 
-// documentedInternal matches guide.md "Internal variables (not part of the operator interface)".
+// documentedInternal matches architecture.md § Configuration "Internal env vars".
 var documentedInternal = []string{
 	"SELFPOST_DATA_DIR",
 	"SELFPOST_DB_PATH",