fix(documents): hoist @napi-rs/canvas so pdfjs can load it in the standalone image

PDF rasterization failed at runtime — pdfjs-dist does a bare
require('@napi-rs/canvas') from its own directory, and the standalone image had
no top-level @napi-rs/canvas for Node's upward resolution to find, so pdfjs
could not polyfill DOMMatrix and every scanned-PDF read fell back to
'PDF scanning needs a Claude vision provider'. Hoist the canvas package and its
musl binary to /app/node_modules/@napi-rs via symlink into the pnpm store (the
prior per-package .node copy only fixed canvas's own loader, not pdfjs's
require). Verified in the container: resolves from pdfjs and renders.

docs/api/openapi.yaml

@@ -1,7 +1,7 @@
 openapi: 3.1.0
 info:
   title: HealthLog API
-  version: 1.28.5
+  version: 1.28.6
   description: >-
     Self-hosted personal-health-tracking PWA — public API surface for the iOS native client and external ingest.