Dashboard cards, backup last-seen

plugins/backups/migrations/versions/0002_backups_lastseenat.py

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+"""backups: record when a configuration was last confirmed unchanged.
+
+Dedup means an unchanged config writes NO revision, so `collectedat` moves only
+when something changes. A machine whose settings have been stable for six months
+therefore has a six-month-old newest revision and is entirely healthy - and
+ShopDB had no way to tell it apart from a machine whose backup stopped running
+six months ago. The evidence that a check happened existed only in a log file on
+the PC.
+
+`lastseenat` records the check rather than the change. It is touched on every
+matching post, including the no-op that writes nothing else.
+
+Backfilled from `collectedat` (falling back to `createdat`) so existing rows
+start from the last moment we can actually prove the config was current, rather
+than from now - claiming a fresh check that never happened would be worse than
+saying nothing.
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = 'backups0002lastseenat'
+down_revision = 'backups0001baseline'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    columns = {c['name'] for c in
+               sa.inspect(op.get_bind()).get_columns('backuprevisions')}
+    if 'lastseenat' not in columns:
+        op.add_column('backuprevisions',
+                      sa.Column('lastseenat', sa.DateTime(), nullable=True))
+        op.execute('UPDATE backuprevisions '
+                   'SET lastseenat = COALESCE(collectedat, createdat)')
+
+
+def downgrade():
+    columns = {c['name'] for c in
+               sa.inspect(op.get_bind()).get_columns('backuprevisions')}
+    if 'lastseenat' in columns:
+        op.drop_column('backuprevisions', 'lastseenat')