[P2] express.json() default 100 kB limit can reject large config imports

[eboudrant]

Priority: P2 · Review finding: #16

src/handler.js createApp uses express.json() with the default 100 kB body limit. A config export with many projects/custom templates can exceed it, making POST /api/config/import fail with a 413 that the UI surfaces as a generic import error.

Fix: raise the limit explicitly (e.g. express.json({ limit: '5mb' })).