fix: publish availability as JSON to fix EasyMQTT parse error

Plain string 'online'/'offline' caused SyntaxError in EasyMQTT which
wildcard-subscribes and JSON.parses all messages. Switch to
{"state":"online"} payload with availability_template in HA discovery.

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

src/mqtt/discovery.js

@@ -3,6 +3,12 @@ const { getClient, getPrefix } = require('./client');
 const { allInputs, allOutputs } = require('../registry');
 const { getInputType } = require('../input-types');
 
+// Availability payload is JSON so EasyMQTT (wildcard subscriber) can parse it.
+// HA uses availability_template to extract the state field.
+const AVAIL_ONLINE  = JSON.stringify({ state: 'online' });
+const AVAIL_OFFLINE = JSON.stringify({ state: 'offline' });
+const AVAIL_TEMPLATE = '{{ value_json.state }}';
+
 function publishAll() {
   const client = getClient();
   if (!client) return;
@@ -19,6 +25,9 @@ function publishAll() {
       state_topic:           stateTopic,
       value_template:        '{{ value_json.value }}',
       availability_topic:    availTopic,
+      availability_template: AVAIL_TEMPLATE,
+      payload_available:     'online',
+      payload_not_available: 'offline',
       device: {
         identifiers:  [`myprimal_${inp.moduleId}`],
         name:         inp.moduleName,
@@ -50,6 +59,9 @@ function publishAll() {
       state_on:              '1',
       state_off:             '0',
       availability_topic:    availTopic,
+      availability_template: AVAIL_TEMPLATE,
+      payload_available:     'online',
+      payload_not_available: 'offline',
       device: {
         identifiers:  [`myprimal_${out.moduleId}`],
         name:         out.moduleName,
@@ -61,16 +73,28 @@ function publishAll() {
     client.publish(configTopic, JSON.stringify(payload), { retain: true });
   }
 
-  // Publish availability online for all modules
+  // Publish availability for all modules (JSON payload)
   const seen = new Set();
   for (const inp of allInputs()) {
     if (!seen.has(inp.moduleSlug)) {
       seen.add(inp.moduleSlug);
-      getClient().publish(`${prefix}/${inp.moduleSlug}/availability`, 'online', { retain: true });
+      client.publish(`${prefix}/${inp.moduleSlug}/availability`, AVAIL_ONLINE, { retain: true });
+    }
+  }
+  for (const out of allOutputs()) {
+    if (!seen.has(out.moduleSlug)) {
+      seen.add(out.moduleSlug);
+      client.publish(`${prefix}/${out.moduleSlug}/availability`, AVAIL_ONLINE, { retain: true });
     }
   }
 
   console.log('[discovery] HA autodiscovery published');
 }
 
-module.exports = { publishAll };
+function publishOffline(moduleSlug) {
+  const client = getClient();
+  if (!client) return;
+  client.publish(`${getPrefix()}/${moduleSlug}/availability`, AVAIL_OFFLINE, { retain: true });
+}
+
+module.exports = { publishAll, publishOffline };