fix: proxy cover art through server to satisfy CSP img-src 'self'

The new CSP blocks direct browser requests to external image origins
(themoviedb.org, thetvdb.com, etc.) used for poster art.

- dashboard.js: add GET /api/dashboard/cover-art?url=... proxy endpoint
  (auth-required, http/https only, image content-type validated, 5MB cap,
  24h Cache-Control, streams response directly to client)
- app.js: route coverArt src through /api/dashboard/cover-art proxy
- server/utils/logger.js: fix hardcoded /app/server.log path (use DATA_DIR)

server/routes/dashboard.js

@@ -721,4 +721,41 @@ router.get('/status', requireAuth, (req, res) => {
   }
 });
 
+// Cover art proxy — fetches external poster images server-side so the
+// browser loads them from 'self' and the CSP img-src stays tight.
+// Requires authentication. Only proxies http/https URLs.
+router.get('/cover-art', requireAuth, async (req, res) => {
+  const { url } = req.query;
+  if (!url || typeof url !== 'string') {
+    return res.status(400).json({ error: 'Missing url parameter' });
+  }
+  let parsed;
+  try {
+    parsed = new URL(url);
+  } catch {
+    return res.status(400).json({ error: 'Invalid url' });
+  }
+  if (parsed.protocol !== 'http:' && parsed.protocol !== 'https:') {
+    return res.status(400).json({ error: 'Only http/https URLs are supported' });
+  }
+  try {
+    const response = await axios.get(url, {
+      responseType: 'stream',
+      timeout: 8000,
+      maxContentLength: 5 * 1024 * 1024 // 5 MB max
+    });
+    const contentType = response.headers['content-type'] || 'image/jpeg';
+    // Only proxy image content types
+    if (!contentType.startsWith('image/')) {
+      return res.status(400).json({ error: 'Remote URL is not an image' });
+    }
+    res.setHeader('Content-Type', contentType);
+    res.setHeader('Cache-Control', 'public, max-age=86400'); // 24h browser cache
+    res.setHeader('X-Content-Type-Options', 'nosniff');
+    response.data.pipe(res);
+  } catch (err) {
+    res.status(502).json({ error: 'Failed to fetch cover art' });
+  }
+});
+
 module.exports = router;