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.

server/app/models.py

@@ -19,7 +19,7 @@ from __future__ import annotations
 
 import time
 
-from sqlalchemy import JSON, Boolean, Float, ForeignKey, Index, Integer, String
+from sqlalchemy import JSON, Boolean, Float, ForeignKey, Index, Integer, LargeBinary, String
 from sqlalchemy.ext.mutable import MutableList
 from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column, relationship
 
@@ -68,6 +68,18 @@ class User(Base):
     # add, without hitting the CalDAV server on every page load.
     calendar_caldav_calendars: Mapped[list | None] = mapped_column(JSON, nullable=True, default=None)
     calendar_caldav_checked_at: Mapped[float] = mapped_column(Float, default=0.0)
+    # WebDAV credentials for whiteboard frame mode (see webdav_client.py,
+    # whiteboard.py) -- generic WebDAV, not Nextcloud-specific, but
+    # webdav_reuse_caldav_creds is a convenience for the common case
+    # where it IS the same Nextcloud account as calendar_caldav_*: skip
+    # re-entering the same username/password, since Nextcloud's CalDAV
+    # and general-file-WebDAV both sit under the one account. Doesn't
+    # try to be clever and derive the reuse automatically -- an explicit
+    # opt-in, same as everywhere else in this project defaults features
+    # off rather than silently inferring them.
+    webdav_username: Mapped[str] = mapped_column(String, default="")
+    webdav_password: Mapped[str] = mapped_column(String, default="")
+    webdav_reuse_caldav_creds: Mapped[bool] = mapped_column(Boolean, default=False)
     created_at: Mapped[float] = mapped_column(Float, default=time.time)
 
     __table_args__ = (
@@ -102,8 +114,9 @@ class Frame(Base):
     # the migrated legacy frame until its device first reports an id.
     device_id: Mapped[str | None] = mapped_column(String, unique=True, nullable=True)
     name: Mapped[str] = mapped_column(String, default="")
-    # Renderer dispatch seam for future calendar/canva modes -- only
-    # "photos" is registered today (see routers/device.py RENDERERS).
+    # Renderer dispatch seam -- "photos", "calendar", or "whiteboard"
+    # (see routers/device.py RENDERERS/ADVANCE_RENDERERS/BACK_RENDERERS
+    # and routers/common.py FRAME_MODES).
     mode: Mapped[str] = mapped_column(String, default="photos")
     # Whose Immich library this frame pulls from; NULL = unclaimed.
     owner_user_id: Mapped[int | None] = mapped_column(
@@ -238,6 +251,27 @@ class Frame(Base):
     # by due date -- see caldav_client.fetch_tasks.
     calendar_tasks_cached: Mapped[list | None] = mapped_column(JSON, nullable=True, default=None)
 
+    # -- whiteboard mode (see webdav_client.py, whiteboard.py,
+    # routers/device.py's RENDERERS["whiteboard"]) -- a frame-wide
+    # setting like calendar mode's own frame_calendars source, not
+    # personal data, but still owner-gated the same way: only
+    # whiteboard_user_id may point the frame at their own account (see
+    # routers/api_frames.py's api_whiteboard_source), since it's their
+    # credentials being used to fetch it. --
+    whiteboard_user_id: Mapped[int | None] = mapped_column(
+        ForeignKey("users.id", ondelete="SET NULL"), nullable=True
+    )
+    # The specific .whiteboard file's WebDAV URL -- pasted directly, same
+    # idiom as calendar_ics_url, not discovered/browsed (unlike CalDAV's
+    # account-has-several-calendars case, a WebDAV account doesn't need
+    # a picker step here since the user already knows which one file).
+    whiteboard_url: Mapped[str] = mapped_column(String, default="")
+    whiteboard_checked_at: Mapped[float] = mapped_column(Float, default=0.0)
+    # Cached rendered PNG bytes (see whiteboard.fetch_and_render) --
+    # BLOB rather than the JSON columns the rest of this cache-pattern
+    # family uses, since this is binary image data, not JSON-shaped.
+    whiteboard_cached_image: Mapped[bytes | None] = mapped_column(LargeBinary, nullable=True)
+
     # -- state --
     current_asset_id: Mapped[str] = mapped_column(String, default="")
     current_asset_set_at: Mapped[float] = mapped_column(Float, default=0.0)