Add container readiness checks and exclude local state from images

Adapted from MikaelKW/nodecast-tv-plus readiness and container context work.

Co-authored-by: MikaelKW <87399352+MikaelKW@users.noreply.github.com>

server/services/health.js

@@ -0,0 +1,18 @@
+const database = require('../db');
+const { getDb } = require('../db/sqlite');
+
+async function health(req, res) {
+    res.setHeader('Cache-Control', 'no-store');
+    if (!req.app.locals.ready) return res.status(503).json({ status: 'unavailable' });
+    try {
+        await database.checkHealth();
+        getDb().prepare('SELECT 1').get();
+        // Readiness may change while the asynchronous JSON check is running.
+        if (!req.app.locals.ready) return res.status(503).json({ status: 'unavailable' });
+        res.json({ status: 'ok' });
+    } catch {
+        res.status(503).json({ status: 'unavailable' });
+    }
+}
+
+module.exports = health;