immich: upgrade to v2.7.3, tune PG for vector search performance

- Bump immich server + ML from v2.6.3 to v2.7.3
- Increase PG shared_buffers to 2GB (memory 3Gi) to prevent
  clip_index eviction by background jobs
- Switch DB_STORAGE_TYPE to SSD (effective_io_concurrency=200,
  random_page_cost=1.2)
- Add pg_prewarm autoprewarm for warm restarts
- Add postgresql.override.conf via init container for tuning
- Add postStart hook to prewarm vector tables on startup

Search latency: ~1.3s → ~130ms (external), ~60ms (internal)

stacks/immich/main.tf

@@ -15,7 +15,7 @@ locals {
 variable "immich_version" {
   type = string
   # Change me to upgrade
-  default = "v2.6.3"
+  default = "v2.7.3"
 }
 variable "nfs_server" { type = string }
 variable "redis_host" { type = string }