Store published presentations on the writable data volume

The default published-presentations folder resolved to the app dir
(/overleaf/services/web/data/published), which isn't writable by the runtime
user → EACCES on publish. Point it at the Overleaf data volume in the
production config (Path.join(DATA_DIR, 'published') = /var/lib/overleaf/data/
published), alongside compiles/output, where the app user can write (and which
persists when a volume is mounted). Overridable via PUBLISHED_PRESENTATIONS_PATH.

server-ce/config/settings.js

@@ -173,6 +173,12 @@ const settings = {
     clsiCacheDir: Path.join(DATA_DIR, 'cache'),
     // Where to write the output files to disk after running LaTeX
     outputDir: Path.join(DATA_DIR, 'output'),
+    // Where to store published-presentation snapshots served at /p/:token.
+    // Lives on the data volume so it is writable by the app user (and, with a
+    // persistent volume, survives restarts).
+    publishedPresentationsFolder:
+      process.env.PUBLISHED_PRESENTATIONS_PATH ||
+      Path.join(DATA_DIR, 'published'),
   },
 
   // Server Config