Let a tasks widget merge multiple task lists, checkbox+color like calendar

Tasks widgets could only ever point at one CalDAV task list (a radio-
button picker, owner-only). Now they merge any number of included task
lists across every linked user, same checkbox-inclusion + optional
pinned-color shape a calendar widget already has for its calendars --
FrameTaskList mirrors FrameCalendar exactly, down to the same owner-
adds/anyone-mutes permission split (api_widget_task_list_select/
api_widget_task_list_color). Reused calendar_render._event_colors/
_draw_color_bar as-is for the per-task color bar -- a task dict's
owner_display_name/color_index is exactly that function's single-
source fallback shape.

Also added an opt-in "show tasks completed in the last 24 hours"
toggle (TaskWidgetConfig.show_completed): caldav_client.fetch_tasks
now accepts a completed_since cutoff and returns completed VTODOs
(with their completion time) instead of silently dropping them, and
_draw_tasks gives a completed task a filled checkbox + muted text
instead of the normal empty-box/due-date row.

Migration 18 splits the single-source TaskWidgetConfig columns
(added by 17, splitting tasks out of the calendar widget in the first
place) into frame_task_lists, carrying forward each widget's existing
single source as its first included list -- same shape migration 9
used carrying forward frame_calendars' old single opt-in.

Verified live in the browser (desktop + mobile): the new "Included
task lists" + "Recently completed" dialog sections, the show_completed
toggle actually persisting through a real HTTP round-trip, and no
regression in the calendar widget's own "Included calendars" dialog.
Full suite (192 tests, including new merge_tasks/config_save/migration
coverage) passes.

server/app/models.py

@@ -360,8 +360,8 @@ class FrameCalendar(Base):
     Keyed by widget_id, not frame_id -- a frame can hold more than one
     independent calendar widget (see Widget), each with its own included-
     calendars set; "included on this frame" stopped being unambiguous
-    the moment that became possible (see migration.py's _migration_17,
-    which re-keyed this table)."""
+    the moment that became possible (see migration.py's
+    _ensure_frame_calendars_rekeyed, which re-keyed this table)."""
 
     __tablename__ = "frame_calendars"
 
@@ -386,6 +386,32 @@ class FrameCalendar(Base):
     )
 
 
+class FrameTaskList(Base):
+    """One CalDAV task list included on one tasks widget -- calendar_key
+    is "caldav:<href>" (an entry in User.calendar_caldav_calendars; no
+    "ics" variant, unlike FrameCalendar -- a plain ICS subscription has
+    no VTODO collection). Same owner-controls-their-own-data shape as
+    FrameCalendar in every other respect: a row is only ever created by
+    its own owner, but any user linked to the frame may flip included
+    back to False, and only the owner may flip it back to True or set
+    color_index. See routers/api_widgets.py's api_widget_task_list_select/
+    api_widget_task_list_color."""
+
+    __tablename__ = "frame_task_lists"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    widget_id: Mapped[int] = mapped_column(ForeignKey("widgets.id", ondelete="CASCADE"))
+    user_id: Mapped[int] = mapped_column(ForeignKey("users.id", ondelete="CASCADE"))
+    calendar_key: Mapped[str] = mapped_column(String)
+    calendar_label: Mapped[str] = mapped_column(String, default="")
+    included: Mapped[bool] = mapped_column(Boolean, default=True)
+    color_index: Mapped[int | None] = mapped_column(Integer, nullable=True, default=None)
+
+    __table_args__ = (
+        Index("ix_frame_task_lists_unique", "widget_id", "user_id", "calendar_key", unique=True),
+    )
+
+
 class Widget(Base):
     """One placed/sized content item on a frame's panel -- the unit the
     widget system replaces the old single Frame.mode with (see
@@ -485,24 +511,27 @@ class TaskWidgetConfig(Base):
     old bolted-on version, the widget's mere presence on the grid is the
     on/off switch, same as every other widget type.
 
-    CalDAV only (a task list is a VTODO collection, not something a
-    plain ICS subscription meaningfully has); source is one specific
-    linked user's own CalDAV calendar, same owner-controls-their-own-
-    data permission split as FrameCalendar. user_id SET NULL on the
-    user's deletion clears the source rather than leaving a dangling
-    reference (checked_at isn't reset by that, but the next refresh
-    attempt finds no source and just returns [])."""
+    Which task lists feed this widget lives in FrameTaskList, not here
+    -- a widget can merge more than one person's list, mirroring
+    CalendarWidgetConfig/FrameCalendar exactly (this used to be a single
+    user_id/calendar_key pair here, one list only; migration 18 carried
+    each widget's existing single source forward as its first
+    FrameTaskList row when splitting this out)."""
 
     __tablename__ = "task_widget_configs"
 
     widget_id: Mapped[int] = mapped_column(ForeignKey("widgets.id", ondelete="CASCADE"), primary_key=True)
-    user_id: Mapped[int | None] = mapped_column(ForeignKey("users.id", ondelete="SET NULL"), nullable=True)
-    calendar_key: Mapped[str | None] = mapped_column(String, nullable=True)
     checked_at: Mapped[float] = mapped_column(Float, default=0.0)
-    # [{"summary", "due" (ISO date/datetime string or None)}, ...],
-    # already filtered to outstanding (not-completed) tasks and sorted
-    # by due date -- see caldav_client.fetch_tasks.
+    # [{"summary", "due", "completed_at" (ISO date/datetime strings or
+    # None), "owner_display_name", "color_index"}, ...] -- the merged
+    # multi-list result, same general shape as CalendarWidgetConfig.
+    # cached_events. See caldav_client.merge_tasks.
     cached: Mapped[list | None] = mapped_column(JSON, nullable=True, default=None)
+    # Also include tasks completed in the last 24h (drawn checked-box +
+    # muted, see calendar_render._draw_tasks) rather than just
+    # outstanding ones -- off by default, same "opt into more" posture
+    # as calendar_weather_enabled.
+    show_completed: Mapped[bool] = mapped_column(Boolean, default=False)
 
 
 class WhiteboardWidgetConfig(Base):