NeuronModule reads NEURON_DATA_DIR to construct the DB path as
$NEURON_DATA_DIR/neuron.db. Without this env var the app falls
back to a temp file, losing all data on pod restart. Also renamed
the PVC file from neuron-prod.db to neuron.db to match.
Completes the three-env setup (dev was added previously). Each env has
deployments, services, ingress, PVC, ConfigMap, and ExternalSecret pulling
from Vault. Prod has larger resource limits and 10Gi PVC. Vault secret at
secret/neuron-technologies/prod seeded separately.