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

vhs/crop/main.go

@@ -0,0 +1,123 @@
+// Command crop trims the uniform background border around each VHS screenshot so the
+// content fills the frame instead of floating in a large empty terminal. Sparse views
+// (alerts, detail, nodes) would otherwise sit in a sea of dead space.
+//
+// Usage: crop [dir]   (dir defaults to vhs/screenshots)
+package main
+
+import (
+	"fmt"
+	"image"
+	"image/color"
+	"image/png"
+	"os"
+	"path/filepath"
+)
+
+// pad is the margin (px) left around the detected content. tol is the per-channel
+// colour distance (summed) above which a pixel counts as content rather than background.
+const (
+	pad = 24
+	tol = 28
+)
+
+func main() {
+	dir := "vhs/screenshots"
+	if len(os.Args) > 1 {
+		dir = os.Args[1]
+	}
+	paths, err := filepath.Glob(filepath.Join(dir, "*.png"))
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "glob: %v\n", err)
+		os.Exit(1)
+	}
+	if len(paths) == 0 {
+		fmt.Fprintf(os.Stderr, "no PNGs in %s\n", dir)
+		os.Exit(1)
+	}
+	for _, p := range paths {
+		w, h, err := cropFile(p)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "crop %s: %v\n", p, err)
+			os.Exit(1)
+		}
+		fmt.Printf("cropped %s -> %dx%d\n", filepath.Base(p), w, h)
+	}
+}
+
+func cropFile(path string) (int, int, error) {
+	f, err := os.Open(path) //nolint:gosec // dev tool: paths come from a trusted local glob
+	if err != nil {
+		return 0, 0, err
+	}
+	src, err := png.Decode(f)
+	_ = f.Close()
+	if err != nil {
+		return 0, 0, err
+	}
+
+	b := src.Bounds()
+	// Background colour sampled from a corner — always inside VHS's blank padding.
+	bgR, bgG, bgB := rgb(src.At(b.Min.X+2, b.Min.Y+2))
+
+	minX, minY := b.Max.X, b.Max.Y
+	maxX, maxY := b.Min.X, b.Min.Y
+	found := false
+	for y := b.Min.Y; y < b.Max.Y; y++ {
+		for x := b.Min.X; x < b.Max.X; x++ {
+			r, g, bl := rgb(src.At(x, y))
+			if abs(r-bgR)+abs(g-bgG)+abs(bl-bgB) > tol {
+				found = true
+				minX, minY = min(minX, x), min(minY, y)
+				maxX, maxY = max(maxX, x), max(maxY, y)
+			}
+		}
+	}
+	if !found {
+		return b.Dx(), b.Dy(), nil // blank frame — leave untouched
+	}
+
+	minX = clamp(minX-pad, b.Min.X, b.Max.X)
+	minY = clamp(minY-pad, b.Min.Y, b.Max.Y)
+	maxX = clamp(maxX+pad+1, b.Min.X, b.Max.X)
+	maxY = clamp(maxY+pad+1, b.Min.Y, b.Max.Y)
+
+	dst := image.NewRGBA(image.Rect(0, 0, maxX-minX, maxY-minY))
+	for y := minY; y < maxY; y++ {
+		for x := minX; x < maxX; x++ {
+			dst.Set(x-minX, y-minY, src.At(x, y))
+		}
+	}
+
+	out, err := os.Create(path) //nolint:gosec // dev tool: paths come from a trusted local glob
+	if err != nil {
+		return 0, 0, err
+	}
+	defer out.Close() //nolint:errcheck // best-effort close on write path
+	if err := png.Encode(out, dst); err != nil {
+		return 0, 0, err
+	}
+	return dst.Bounds().Dx(), dst.Bounds().Dy(), nil
+}
+
+func rgb(c color.Color) (int, int, int) {
+	r, g, b, _ := c.RGBA()
+	return int(r >> 8), int(g >> 8), int(b >> 8)
+}
+
+func abs(x int) int {
+	if x < 0 {
+		return -x
+	}
+	return x
+}
+
+func clamp(v, lo, hi int) int {
+	if v < lo {
+		return lo
+	}
+	if v > hi {
+		return hi
+	}
+	return v
+}