Add whiteboard frame mode (Nextcloud Whiteboard / Excalidraw over WebDAV)

New third mode alongside photos/calendar: fetches a .whiteboard file
over plain WebDAV (Basic auth -- generic, not Nextcloud-specific) and
renders it via a small Node.js sidecar using Excalidraw's own real
export code (@excalidraw/utils + @resvg/resvg-js, no headless browser),
since a .whiteboard file turns out to be Excalidraw scene JSON, not an
image. The sidecar runs as a second process inside this same container
(Dockerfile installs Node, start.sh backgrounds it before exec'ing
uvicorn) rather than a separate docker-compose service -- lightweight,
stateless, reachable only at 127.0.0.1 from the Python process, nothing
worth independently scaling.

The rendered PNG is treated exactly like a photo from there on --
composed/quantized through the existing image_pipeline (letterboxed,
never cropped) rather than a second parallel rendering pipeline.

WebDAV credentials support the common "it's actually the same Nextcloud
account as my CalDAV" case (an explicit opt-in checkbox, not silently
inferred) while still working with any WebDAV server generically.
Frame-level source (URL + owning account) follows the same owner-
controls-their-own-data permission split as calendar sources and the
week view's task list: only the account owner can point a frame at it,
anyone linked can clear it.

Honest limitation: this environment has no Node.js/npm, so
render-service/ is written carefully against each library's documented
API (verified via the npm registry, including transitive dependency
licenses after the CalDAV/AGPL surprise earlier this session) but has
never actually been executed. First real docker build is the first
true test -- see render-service/README.md.

.gitignore

@@ -17,6 +17,12 @@ server/**/__pycache__/
 server/.venv/
 server/*.egg-info/
 server/data/
+# render-service/ (whiteboard mode's Node sidecar) -- installed fresh
+# inside the Docker image, never committed. No package-lock.json exists
+# yet either (no Node/npm available in this project's dev environment to
+# generate one -- see render-service/README.md); if one's added later, do
+# NOT ignore it, lockfiles belong in git.
+server/render-service/node_modules/
 # Real deploy config, copied from docker-compose.yml.example -- holds the
 # Immich API key, must never be committed.
 server/docker-compose.yml