refactor(cuprate): one CUPRATE_MIN_DISK_GB, manifest matches it (review)

450 existed as two independent Rust constants (RPC gates vs boot
reconciler) linked only by a "keep in lockstep" comment — updating one
would reopen the disk-fill hole. Move it to crate::constants as the
single source of truth both paths import.

Also raise apps/cuprate/manifest.yml storage dependency and disk_limit
from 300Gi to 450Gi so manifest-driven surfaces (store size, pre-checks)
show the number the gate actually enforces — a user provisioning to the
displayed 300 was refused at an unexplained 450. Catalog regenerated
(cuprate entry re-embedded; still unsigned pending sign-catalog.sh).

releases/app-catalog.json

@@ -1324,13 +1324,13 @@
           },
           "dependencies": [
             {
-              "storage": "300Gi"
+              "storage": "450Gi"
             }
           ],
           "resources": {
             "cpu_limit": 0,
             "memory_limit": "10Gi",
-            "disk_limit": "300Gi"
+            "disk_limit": "450Gi"
           },
           "security": {
             "capabilities": [],