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>

corrosion-host-agent/src/config.rs

@@ -34,6 +34,12 @@ pub struct AgentSection {
     pub license_id: Option<String>,
     pub nats_url: Option<String>,
     pub nats_token: Option<String>,
+    /// NATS username for per-license auth. Defaults to license_id when a
+    /// password is set but no user is given.
+    pub nats_user: Option<String>,
+    /// NATS password (the per-license token). When set, the agent authenticates
+    /// with user+password instead of a bare token.
+    pub nats_password: Option<String>,
     #[serde(default = "default_heartbeat_seconds")]
     pub heartbeat_seconds: u64,
     #[serde(default = "default_log_level")]
@@ -122,6 +128,8 @@ pub struct Settings {
     pub license_id: String,
     pub nats_url: String,
     pub nats_token: Option<String>,
+    pub nats_user: Option<String>,
+    pub nats_password: Option<String>,
     pub heartbeat_seconds: u64,
     pub log_level: String,
     pub instances: Vec<InstanceConfig>,
@@ -167,6 +175,16 @@ fn resolve(file: ConfigFile) -> Result<Settings> {
         .filter(|v| !v.is_empty())
         .or(file.agent.nats_token);
 
+    let nats_user = std::env::var("CORROSION_NATS_USER")
+        .ok()
+        .filter(|v| !v.is_empty())
+        .or(file.agent.nats_user);
+
+    let nats_password = std::env::var("CORROSION_NATS_PASSWORD")
+        .ok()
+        .filter(|v| !v.is_empty())
+        .or(file.agent.nats_password);
+
     validate_subject_segment("license_id", &license_id)?;
 
     let mut seen: HashSet<&str> = HashSet::new();
@@ -196,6 +214,8 @@ fn resolve(file: ConfigFile) -> Result<Settings> {
         license_id,
         nats_url,
         nats_token,
+        nats_user,
+        nats_password,
         heartbeat_seconds: file.agent.heartbeat_seconds,
         log_level: file.agent.log_level,
         instances: file.instances,