migrate all secrets from SOPS to Vault KV

- Add vault provider to root terragrunt.hcl (generated providers.tf)
- Delete stacks/vault/vault_provider.tf (now in generated providers.tf)
- Add 124 variable declarations + 43 vault_kv_secret_v2 resources to
  vault/main.tf to populate Vault KV at secret/<stack-name>
- Migrate 43 consuming stacks to read secrets from Vault KV via
  data "vault_kv_secret_v2" instead of SOPS var-file
- Add dependency "vault" to all migrated stacks' terragrunt.hcl
- Complex types (maps/lists) stored as JSON strings, decoded with
  jsondecode() in locals blocks

Bootstrap secrets (vault_root_token, vault_authentik_client_id,
vault_authentik_client_secret) remain in SOPS permanently.

Apply order: vault stack first (populates KV), then all others.

stacks/plotting-book/main.tf

@@ -2,10 +2,6 @@ variable "tls_secret_name" {
   type      = string
   sensitive = true
 }
-variable "plotting_book_session_secret" {
-  type      = string
-  sensitive = true
-}
 variable "plotting_book_google_client_id" {
   type      = string
   sensitive = true
@@ -15,6 +11,10 @@ variable "plotting_book_google_client_secret" {
   sensitive = true
 }
 
+data "vault_kv_secret_v2" "secrets" {
+  mount = "secret"
+  name  = "plotting-book"
+}
 
 resource "kubernetes_namespace" "plotting-book" {
   metadata {
@@ -92,7 +92,7 @@ resource "kubernetes_deployment" "plotting-book" {
           image_pull_policy = "Always"
           env {
             name  = "SESSION_SECRET"
-            value = var.plotting_book_session_secret
+            value = data.vault_kv_secret_v2.secrets.data["session_secret"]
           }
           env {
             name  = "GOOGLE_CLIENT_ID"

stacks/plotting-book/terragrunt.hcl

@@ -6,3 +6,8 @@ dependency "platform" {
   config_path  = "../platform"
   skip_outputs = true
 }
+
+dependency "vault" {
+  config_path  = "../vault"
+  skip_outputs = true
+}