Split the tasks feature out of the calendar widget into its own widget type

Task lists used to be a week-view-only sub-feature bolted onto calendar
widgets (CalendarWidgetConfig.tasks_*), so a task list could only exist
tied to a calendar's view and only inside its footprint. Tasks are now
a standalone widget type (TaskWidgetConfig, app/widgets/tasks.py) that
can be placed and sized independently, same as photos/calendar/
whiteboard -- no separate "enabled" flag either, since being on the
grid at all is the on/off switch, matching every other widget type.

Migration 17 creates task_widget_configs, extracts any existing
calendar widget's configured task source into a new sibling tasks
widget (auto-placed in open grid space, source dropped+logged if truly
none is left), then drops calendar_widget_configs' now-dead tasks_*
columns in the same migration -- this project's usual same-migration-
drop convention. Also handles the rarer case of a database jumping
straight from before the widget system existed to after this split in
one boot, via the legacy Frame.calendar_tasks_* columns.

Verified live in the browser at desktop and mobile widths: adding a
Tasks widget, its own dialog (task-list source picker + preview), and
confirming the calendar widget's dialog no longer mentions tasks at
all. Full test suite (180 tests, including new coverage for the widget
render/actions, the migration's data-extraction path, and the
permission-boundary shape for tasks-source) passes.

server/app/models.py

@@ -407,7 +407,7 @@ class Widget(Base):
 
     id: Mapped[int] = mapped_column(primary_key=True)
     frame_id: Mapped[int] = mapped_column(ForeignKey("frames.id", ondelete="CASCADE"))
-    widget_type: Mapped[str] = mapped_column(String)  # "photos" | "calendar" | "whiteboard"
+    widget_type: Mapped[str] = mapped_column(String)  # "photos" | "calendar" | "whiteboard" | "tasks"
     x: Mapped[int] = mapped_column(Integer)
     y: Mapped[int] = mapped_column(Integer)
     w: Mapped[int] = mapped_column(Integer)
@@ -450,9 +450,12 @@ class CalendarWidgetConfig(Base):
     fields that used to live as calendar_* columns directly on Frame,
     minus calendar_photo_inlay (dropped: arbitrary widget placement
     subsumes what a fixed 50/50 inlay split did, so it's not a special
-    case anymore, just place a photo widget alongside). "Included
-    calendars" is its own table (FrameCalendar), widget_id-keyed so each
-    calendar widget on a frame has its own independent set."""
+    case anymore, just place a photo widget alongside) and minus
+    tasks_* (also dropped: split out into its own standalone widget
+    type, see TaskWidgetConfig, so a task list isn't tied to a
+    calendar's week view/footprint anymore). "Included calendars" is
+    its own table (FrameCalendar), widget_id-keyed so each calendar
+    widget on a frame has its own independent set."""
 
     __tablename__ = "calendar_widget_configs"
 
@@ -471,13 +474,35 @@ class CalendarWidgetConfig(Base):
     week_days: Mapped[int] = mapped_column(Integer, default=7)
     week_layout: Mapped[str] = mapped_column(String, default="horizontal")
     week_start_offset: Mapped[int] = mapped_column(Integer, default=0)
-    tasks_enabled: Mapped[bool] = mapped_column(Boolean, default=False)
-    tasks_user_id: Mapped[int | None] = mapped_column(
-        ForeignKey("users.id", ondelete="SET NULL"), nullable=True
-    )
-    tasks_calendar_key: Mapped[str | None] = mapped_column(String, nullable=True)
-    tasks_checked_at: Mapped[float] = mapped_column(Float, default=0.0)
-    tasks_cached: Mapped[list | None] = mapped_column(JSON, nullable=True, default=None)
+
+
+class TaskWidgetConfig(Base):
+    """One tasks widget's settings + cached-fetch state -- split out of
+    CalendarWidgetConfig (which used to carry these as tasks_* columns,
+    a week-view-only task list bolted onto a calendar widget) so a task
+    list can be placed and sized on its own, independent of any
+    calendar's view/footprint. No separate "enabled" flag -- unlike the
+    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 [])."""
+
+    __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.
+    cached: Mapped[list | None] = mapped_column(JSON, nullable=True, default=None)
 
 
 class WhiteboardWidgetConfig(Base):
@@ -500,6 +525,7 @@ WIDGET_CONFIG_MODELS: dict[str, type] = {
     "photos": PhotoWidgetConfig,
     "calendar": CalendarWidgetConfig,
     "whiteboard": WhiteboardWidgetConfig,
+    "tasks": TaskWidgetConfig,
 }