chore: add TUI screenshots via VHS with realistic seed data

Screenshots capture 4 views: monitors dashboard (hero), detail panel,
alerts tab, and logs tab. Seed data uses homelab-themed monitors with
a SQL backfill for rich sparkline history, state changes, and log
entries.

Also fixes latencySparkline to color DOWN checks red instead of green
— previously failed checks with 0ms latency rendered as green bars.

vhs/demo.tape

@@ -0,0 +1,54 @@
+Set Shell "bash"
+Set Width 1400
+Set Height 800
+Set FontSize 14
+Set Padding 20
+Set Framerate 15
+Set TypingSpeed 50ms
+
+Hide
+Type "bash vhs/setup.sh /tmp/uptop-vhs.db"
+Enter
+Sleep 45s
+Show
+Sleep 5s
+
+# Sites tab — hero shot with mixed monitor states
+Screenshot vhs/screenshots/monitors.png
+Sleep 1s
+
+# Navigate to Nextcloud (row 6: group + 3 children + Auth Portal)
+Down
+Sleep 200ms
+Down
+Sleep 200ms
+Down
+Sleep 200ms
+Down
+Sleep 200ms
+Down
+Sleep 200ms
+Type "i"
+Sleep 3s
+Screenshot vhs/screenshots/detail.png
+Sleep 1s
+
+# Close detail
+Escape
+Sleep 1s
+
+# Tab to Alerts
+Tab
+Sleep 2s
+Screenshot vhs/screenshots/alerts.png
+Sleep 1s
+
+# Tab to Logs
+Tab
+Sleep 2s
+Screenshot vhs/screenshots/logs.png
+Sleep 1s
+
+# Quit
+Type "q"
+Sleep 1s