infra: untrack generated backend.tf (stale PG creds + .200 literal) [CI SKIP]

terragrunt generates backend.tf per run (remote_state generate,
if_exists=overwrite_terragrunt) from get_env("PG_CONN_STR"); these 72 committed
copies are stale artifacts already covered by .gitignore:65. They held a
plaintext (Vault-rotated, ~expired) PG password + the .200 state-backend literal
and were re-committed by CI on every run. git rm --cached stops that; they
regenerate locally from PG_CONN_STR. The live .200:5432 literal now lives only
in scripts/tg (its single bootstrap source).

Part of the L4 LB-IP review (docs/plans/2026-06-03-lb-ip-hygiene-design.md).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

stacks/platform/backend.tf

@@ -1,6 +0,0 @@
-# Generated by Terragrunt. Sig: nIlQXj57tbuaRZEa
-terraform {
-  backend "local" {
-    path = "/home/wizard/code/infra/state/stacks/platform/terraform.tfstate"
-  }
-}