Widget system Phase 1: per-type render/action modules

New app/widgets/ package (photos.py, calendar.py, whiteboard.py, plus
the WIDGET_TYPES registry) -- the widget-system analogue of
routers/device.py's old RENDERERS/ADVANCE_RENDERERS/BACK_RENDERERS,
generalized from "one mode owns the whole panel" to "each widget renders
into its own region and responds to named button actions." Each module
exposes render(db, frame, widget, target_w, target_h) -> Image.Image
(never raises -- a widget's own fetch hiccup falls back to a small
placeholder rather than taking the whole panel's render down) and an
ACTIONS registry for NEXT/BACK button assignment.

Supporting changes needed to give the widget modules something to call,
all mechanical/behavior-preserving for every existing caller:

- image_pipeline.py: render_panel(regions, ...) generalizes render_frame's
  tail (paste, enhance once, overlay once, quantize once, pack once) from
  one photo to N regions -- not a restructuring, since calendar mode's
  photo-inlay feature already pastes a second composed image onto the
  canvas before that single shared pipeline runs.
- photo_queue.py: advance_forced/back_forced/remove_from_rotation/
  get_current take an explicit `frame` param now that `cfg` won't always
  be the Frame itself once photo-queue state moves to PhotoWidgetConfig
  -- caught a real latent bug while doing this: get_current was reading
  refresh_interval_s off `cfg`, but that's a frame-level wake-cadence
  setting, not something that becomes per-widget, so it now reads that
  off `frame` explicitly instead.
- routers/common.py: list_assets/fetch_source_and_faces take album_id/
  display_mode directly instead of a whole Frame (both only ever read
  that one attribute off it); new get_or_refresh_*_for_widget siblings
  of the existing calendar/weather/tasks/whiteboard cache helpers, read/
  writing the new per-widget config tables -- the Frame-scoped originals
  are untouched and still what routers/device.py's actual dispatch calls
  until the Phase 2 cutover.

26 new tests (95 total): render_panel size/placement/orientation
coverage, and per-widget-type render/action tests (unconfigured ->
placeholder, a fetch failure -> placeholder not a crash, actions mutate
the right state). Full suite passes; diff-reviewed to confirm
device.py's actual RENDERERS dispatch and the old Frame-scoped
get_or_refresh_* bodies are unchanged, so this is safe to deploy on its
own despite being step 1 of a two-step cutover (see the project plan on
why the *next* step, not this one, has to ship atomically).

server/app/widgets/whiteboard.py

@@ -0,0 +1,41 @@
+"""Whiteboard widget: fetches/renders a Nextcloud Whiteboard (or any
+WebDAV .whiteboard file) into the widget's own region -- the
+widget-system analogue of routers/device.py's old
+_render_whiteboard_mode/_advance_whiteboard_mode/_back_whiteboard_mode.
+
+No real "next"/"back" concept for a static board (same as before the
+widget system) -- both buttons map to the same "check now" action, a
+forced re-fetch/re-render bypassing the normal throttle."""
+
+from __future__ import annotations
+
+import io
+
+from PIL import Image
+from sqlalchemy.orm import Session
+
+from ..image_pipeline import compose_into
+from ..models import Frame, Widget
+from ..routers.common import get_or_refresh_whiteboard_for_widget
+from ._shared import placeholder_image
+
+ACTION_LABELS = {"check_now": "Check for updates"}
+
+
+def render(db: Session, frame: Frame, widget: Widget, target_w: int, target_h: int) -> Image.Image:
+    png_bytes = get_or_refresh_whiteboard_for_widget(db, frame, widget)
+    if png_bytes is None:
+        return placeholder_image(target_w, target_h, ["Whiteboard widget", "not configured yet"])
+
+    source = Image.open(io.BytesIO(png_bytes)).convert("RGB")
+    # letterbox, never cropped: unlike a photo, losing part of a
+    # whiteboard to a crop loses actual content, not just some background
+    # (see the old _render_whiteboard_mode's identical reasoning).
+    return compose_into(source, faces=None, target_w=target_w, target_h=target_h, display_mode="letterbox")
+
+
+def _check_now(db: Session, frame: Frame, widget: Widget) -> None:
+    get_or_refresh_whiteboard_for_widget(db, frame, widget, force=True)
+
+
+ACTIONS = {"check_now": _check_now}