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/migrations/versions/0021_dashboard_widget_grid.py

@@ -0,0 +1,48 @@
+"""Replace dashboard_widgets.position with a Grafana-style grid (x/y/w/h)
+
+Revision ID: 0021_dashboard_widget_grid
+Revises: 0020_ansible_run_results
+Create Date: 2026-06-17
+"""
+from typing import Sequence, Union
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "0021_dashboard_widget_grid"
+down_revision: Union[str, None] = "0020_ansible_run_results"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # New grid columns (server_default so existing rows are populated; the ORM
+    # supplies these on insert thereafter). 12-col grid, default widget 4x4.
+    op.add_column("dashboard_widgets",
+                  sa.Column("grid_x", sa.Integer, nullable=False, server_default="0"))
+    op.add_column("dashboard_widgets",
+                  sa.Column("grid_y", sa.Integer, nullable=False, server_default="0"))
+    op.add_column("dashboard_widgets",
+                  sa.Column("grid_w", sa.Integer, nullable=False, server_default="4"))
+    op.add_column("dashboard_widgets",
+                  sa.Column("grid_h", sa.Integer, nullable=False, server_default="4"))
+    # Backfill from the old `position` order into a 3-up grid, reproducing the
+    # previous masonry layout: 3 widgets per row, each 4 cols wide x 4 cells tall.
+    op.execute(
+        "UPDATE dashboard_widgets "
+        "SET grid_x = (position % 3) * 4, grid_y = (position / 3) * 4"
+    )
+    op.drop_column("dashboard_widgets", "position")
+
+
+def downgrade() -> None:
+    op.add_column("dashboard_widgets",
+                  sa.Column("position", sa.Integer, nullable=False, server_default="0"))
+    # Reconstruct a linear order from the grid (reading order: top-to-bottom,
+    # then left-to-right within a row).
+    op.execute(
+        "UPDATE dashboard_widgets SET position = grid_y * 3 + (grid_x / 4)"
+    )
+    op.drop_column("dashboard_widgets", "grid_h")
+    op.drop_column("dashboard_widgets", "grid_w")
+    op.drop_column("dashboard_widgets", "grid_y")
+    op.drop_column("dashboard_widgets", "grid_x")