fix(infra): revert to mongo:8, keep cache-size arg removed

mongo:7 can't open data files written by mongo:8 (exit code 62 =
NeedsDowngrade). Stay on mongo:8 — the SIGSEGV was caused by the
--wiredTigerCacheSizeGB=0.25 flag, not the version. Removing the flag
is the actual fix.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

infrastructure/terraform/mongodb.tf

@@ -76,7 +76,7 @@ resource "kubernetes_stateful_set" "mongodb" {
       spec {
         container {
           name  = "mongodb"
-          image = "mongo:7"
+          image = "mongo:8"
 
           env {
             name  = "MONGO_INITDB_ROOT_USERNAME"