chore: TUI screenshots, README polish, changelog rewrite (#32)

- Add 6 TUI screenshots to assets/ (monitors, alerts, logs, nodes, detail, theme)
- Rewrite README with hero image, badges, collapsible install sections
- Rewrite changelog to match actual CalVer tag history
- VHS tooling extracted to lerko/uptop-vhs

Reviewed-on: lerko/uptop#32

internal/store/sqlite.go

@@ -88,6 +88,14 @@ func (d *SQLiteDialect) CreateTablesSQL() []string {
 			changed_at DATETIME DEFAULT CURRENT_TIMESTAMP
 		)`,
 		`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 DATETIME,
+			last_send_ok BOOLEAN DEFAULT 0,
+			last_error TEXT DEFAULT '',
+			send_count INTEGER DEFAULT 0,
+			fail_count INTEGER DEFAULT 0
+		)`,
 	}
 }
 
@@ -113,6 +121,10 @@ func (d *SQLiteDialect) UpsertNodeSQL() string {
 	return "INSERT OR REPLACE INTO nodes (id, name, region, last_seen, version) VALUES (?, ?, ?, CURRENT_TIMESTAMP, ?)"
 }
 
+func (d *SQLiteDialect) UpsertAlertHealthSQL() string {
+	return "INSERT OR REPLACE INTO alert_health (alert_id, last_send_at, last_send_ok, last_error, send_count, fail_count) VALUES (?, ?, ?, ?, ?, ?)"
+}
+
 func (d *SQLiteDialect) ResetSequenceOnEmpty(db *sql.DB, table string) {
 	var count int
 	_ = db.QueryRow("SELECT COUNT(*) FROM " + table).Scan(&count) //nolint:errcheck