feat(dashboard): phase B drag-resize grid (Gridstack) replacing masonry

Milestone 72 phase B — Grafana-style drag-resize grid for dashboard widgets:
- DashboardWidget: replace `position` with a 12-col grid placement
  (grid_x/grid_y/grid_w/grid_h). Migration 0021 backfills the old position
  order into a 3-up grid (4 cols x 4 cells each) and drops position.
- View + share render a static CSS grid from x/y/w/h: fixed cell height
  (h * 70px) with the body scrolling, so the arranged layout is what's shown;
  collapses to a single column under 820px.
- Edit view: Gridstack.js 12.6.0 (vanilla, CDN, pinned) — drag the title bar
  to move, drag a corner/edge to resize; every change autosaves to a new
  /d/<id>/edit/layout endpoint. Replaces the SortableJS position reorder.
- add_widget appends at the bottom-left; remove no longer renumbers.
  _get_widgets now orders by grid position (drives DOM + mobile fallback order).

Note: Gridstack drag-resize is browser-only, so CI can't exercise it — needs an
operator visual check of the edit experience.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

steward/models/dashboard.py

@@ -50,6 +50,11 @@ class DashboardWidget(Base):
         Integer, ForeignKey("dashboards.id", ondelete="CASCADE"), nullable=False,
     )
     widget_key: Mapped[str] = mapped_column(String(64), nullable=False)
-    position: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+    # Grid placement (Grafana-style): 12-column grid, row height in cell units.
+    # x/y are the top-left cell; w/h the span. Replaces the old `position` order.
+    grid_x: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+    grid_y: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+    grid_w: Mapped[int] = mapped_column(Integer, nullable=False, default=4)
+    grid_h: Mapped[int] = mapped_column(Integer, nullable=False, default=4)
     title: Mapped[str | None] = mapped_column(String(128), nullable=True)
     config_json: Mapped[str | None] = mapped_column(Text, nullable=True)