feat(nats): per-license auth mechanism — agent user/password, scoped broker, generator (non-breaking)

Closes the open broker (anonymous publish to any tenant's corrosion.*).
Per-license isolation via NATS user/password + subject permissions:
each license -> user=license_id, password=HMAC-SHA256(license_id,
NATS_TOKEN_SECRET), scoped to corrosion.{license_id}.> + _INBOX. Backend
uses a privileged internal user.

- Agent (alpha.5): nats_user/nats_password config + env, user_and_password
  auth; falls back to token/anonymous (transition-safe)
- Backend: connects with NATS_INTERNAL_USER/PASSWORD when set, else anon
- scripts/generate-nats-auth.mjs: regenerates nats-auth.conf from the
  licenses table; NATS_AUTH_STAGE=open keeps a no_auth_user fallback
  (verify creds first), =enforce rejects anonymous
- committed nats-auth.conf is the SAFE OPEN default (no secrets); the
  host copy carries real users and is not committed
- compose: NATS_INTERNAL_USER/PASSWORD/NATS_TOKEN_SECRET, mount nats-auth.conf

Entirely non-breaking until secrets+config deployed; staged cutover next.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docker/docker-compose.yml

@@ -31,6 +31,9 @@ services:
     volumes:
       - nats_data:/data
       - ./nats.conf:/etc/nats/nats.conf:ro
+      # Per-license authorization (generated on the host; carries secrets, not
+      # committed with real users — see scripts/generate-nats-auth.mjs).
+      - ./nats-auth.conf:/etc/nats/nats-auth.conf:ro
     ports:
       - "8089:4222"   # Client connections
 
@@ -43,6 +46,12 @@ services:
       DATABASE_URL: postgres://corrosion:${DB_PASSWORD:-corrosion_dev}@postgres:5432/corrosion
       DATABASE_MAX_CONNECTIONS: "20"
       NATS_URL: nats://nats:4222
+      # Privileged internal NATS user (full corrosion.> access). Empty = anonymous.
+      NATS_INTERNAL_USER: ${NATS_INTERNAL_USER:-}
+      NATS_INTERNAL_PASSWORD: ${NATS_INTERNAL_PASSWORD:-}
+      # Secret for deriving per-license agent passwords (shared with the
+      # nats-auth generator). HMAC-SHA256(license_id, secret).
+      NATS_TOKEN_SECRET: ${NATS_TOKEN_SECRET:-}
       JWT_SECRET: ${JWT_SECRET}
       JWT_ACCESS_EXPIRY_SECONDS: "14400"
       JWT_REFRESH_EXPIRY_SECONDS: "604800"