feat(sv): Backups card — offsite DR status (Core-4 -> Farm) + /api/backups (2.6.0)

migration 026 backup_runs; POST ingest (owner) from offsite-backup.sh, GET for the
Sacred Valley card showing last run, per-guest sizes, Farm free, schedule.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/db/migrations/026_backup_runs.sql

@@ -0,0 +1,12 @@
+-- 026_backup_runs.sql
+-- Offsite DR backup run history, fed by /usr/local/bin/offsite-backup.sh on CT 300
+-- (Core-4 vzdump -> Farm/Won). Powers the Sacred Valley "Backups" card.
+CREATE TABLE IF NOT EXISTS backup_runs (
+  id             serial PRIMARY KEY,
+  ran_at         timestamptz NOT NULL DEFAULT now(),
+  ok             boolean NOT NULL DEFAULT true,
+  total_bytes    bigint,
+  won_free_bytes bigint,
+  guests         jsonb,            -- [{vmid,name,bytes}]
+  duration_sec   integer
+);