add postiz + instagram-poster stacks for IG Stories pipeline

New stacks:
- stacks/postiz/ — Postiz scheduler (Helm chart v1.0.5, image v2.21.7)
  with bundled PG/Redis, /uploads PVC on proxmox-lvm, JWT_SECRET
  via ESO from secret/instagram-poster.
- stacks/instagram-poster/ — custom Python service that polls Immich
  for the 'instagram' tag, reformats photos to 9:16 with blurred-bg
  letterbox, exposes /image/<asset_id> publicly so Postiz can fetch.
  Image: forgejo.viktorbarzin.me/viktor/instagram-poster.

n8n: 3 new workflows (discover, approval, post) for the Telegram
inline-button approval UX. Adds ExternalSecret + env vars for
TELEGRAM_BOT_TOKEN, TELEGRAM_CHAT_ID, IMMICH_API_KEY, plus static
URLs for the new service.

Vault: seed secret/instagram-poster with telegram_bot_token,
telegram_chat_id, immich_api_key, postiz_api_token,
postiz_jwt_secret before applying.

stacks/postiz/terragrunt.hcl

@@ -0,0 +1,13 @@
+include "root" {
+  path = find_in_parent_folders()
+}
+
+dependency "platform" {
+  config_path  = "../platform"
+  skip_outputs = true
+}
+
+dependency "vault" {
+  config_path  = "../vault"
+  skip_outputs = true
+}