fix(infra): switch MongoDB to v7 to fix ARM64 segfault

mongo:8 was crashing with exit code 139 (SIGSEGV) on the Hetzner CAX11
ARM64 instance. Switch to mongo:7 (LTS) which has more stable ARM64
support. Also remove the --wiredTigerCacheSizeGB=0.25 arg since the
512Mi memory limit already bounds memory use adequately.

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

infrastructure/terraform/mongodb.tf

@@ -76,8 +76,7 @@ resource "kubernetes_stateful_set" "mongodb" {
       spec {
         container {
           name  = "mongodb"
-          image = "mongo:8"
-          args  = ["--wiredTigerCacheSizeGB=0.25"]
+          image = "mongo:7"
 
           env {
             name  = "MONGO_INITDB_ROOT_USERNAME"