chore(tui): polish demo + regenerate screenshots

Rework the VHS demo so the README screenshots actually entice a download.

Demo data / tooling:
- seed.yaml: real, reachable service URLs (detail now shows nextcloud.com,
  not example.com); Auth Portal -> non-resolving home.arpa host so it reads
  as a believable, reliably-DOWN monitor
- backfill: transient outages for Nextcloud/Jellyfin/Immich aligned with their
  state changes (uptime % now matches); log timestamps derived from now so the
  Logs view reads chronologically; real SSL warning; three probe nodes across
  regions; seeded alert send health
- demo.tape: shorter warm-up, added Nodes + theme captures, ordered so every
  shot stays inside the 60s node-freshness window (consistent probe count)
- vhs/crop: new tool to trim the empty terminal border around each screenshot
- setup.sh: build backfill up front for deterministic timing; UPTOP_DEMO=1

Supporting code:
- persist alert send health (new alert_health table, load on startup,
  best-effort save on send) so health/last-sent survive restarts
- latency Min/Avg/Max ignore failed checks (no more "Min 0ms")
- correct "probe"/"probes" pluralization
- stable status dot instead of an animated spinner under UPTOP_DEMO

internal/store/postgres.go

@@ -81,6 +81,14 @@ func (d *PostgresDialect) CreateTablesSQL() []string {
 			changed_at TIMESTAMP DEFAULT NOW()
 		)`,
 		`CREATE INDEX IF NOT EXISTS idx_state_changes_site ON state_changes(site_id, changed_at DESC)`,
+		`CREATE TABLE IF NOT EXISTS alert_health (
+			alert_id INTEGER PRIMARY KEY,
+			last_send_at TIMESTAMP,
+			last_send_ok BOOLEAN DEFAULT FALSE,
+			last_error TEXT DEFAULT '',
+			send_count INTEGER DEFAULT 0,
+			fail_count INTEGER DEFAULT 0
+		)`,
 	}
 }
 
@@ -106,6 +114,10 @@ func (d *PostgresDialect) UpsertNodeSQL() string {
 	return "INSERT INTO nodes (id, name, region, last_seen, version) VALUES ($1, $2, $3, NOW(), $4) ON CONFLICT (id) DO UPDATE SET name = EXCLUDED.name, region = EXCLUDED.region, last_seen = NOW(), version = EXCLUDED.version"
 }
 
+func (d *PostgresDialect) UpsertAlertHealthSQL() string {
+	return "INSERT INTO alert_health (alert_id, last_send_at, last_send_ok, last_error, send_count, fail_count) VALUES ($1, $2, $3, $4, $5, $6) ON CONFLICT (alert_id) DO UPDATE SET last_send_at = EXCLUDED.last_send_at, last_send_ok = EXCLUDED.last_send_ok, last_error = EXCLUDED.last_error, send_count = EXCLUDED.send_count, fail_count = EXCLUDED.fail_count"
+}
+
 func (d *PostgresDialect) ResetSequenceOnEmpty(db *sql.DB, table string) {}
 
 func (d *PostgresDialect) ImportWipe(tx *sql.Tx) {