feat(vault): erp prod runtime may read the shared GCS backup creds (kv_read_paths)

Adds an optional kv_read_paths list to the app_policy module (default []) so an
app's env=prod runtime policy can read extra kvv2 data paths — e.g. a shared
backup-creds path owned by another app. Plumbed through the root applications
schema + module call (dynamic rule, read+list).

Set for erp: kv_read_paths = ["kvv2/data/longhorn/gcs-backup"], so the dedicated
Dolibarr backup CronJob (erp chart, gated) can read the existing GCS HMAC creds
via its own VaultStaticSecret instead of borrowing the Longhorn secret
cross-namespace or duplicating credentials.

No-op for every other app (default []). Only the `erp` runtime policy gains one
read+list rule.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

hashicorp-vault/iac/variables.tf

@@ -19,5 +19,8 @@ variable "applications" {
     # every existing app — backwards compatible by the elision rule. Non-prod envs
     # produce additional runtime policies named "<name>-<env>".
     envs = optional(list(string), ["prod"])
+    # Extra kvv2 data paths the app's prod runtime policy may read (read,list) —
+    # e.g. a shared backup-creds path owned by another app. Default none.
+    kv_read_paths = optional(list(string), [])
   }))
 }