feat: add emoji reactions, light/dark themes and rollout observability

Add a set of guest-facing and operator-facing features that make the
deployment-strategy demos (recreate, rolling, canary) more visual:

- Emoji reactions (heart/tada/thumbsup/smile): visitors react without an
  account, a first-party cookie keeps counts honest, the tally streams
  live to every tab over SSE. Backed by a new reactions table with an
  idempotent upsert; whitelisted emoji so the endpoint can't be abused.
- Light & dark themes: the wall follows the OS preference, visitors can
  toggle it (persisted locally), and a moderator can force a theme for
  everyone from the admin console, pushed live over SSE.
- Per-replica request counter: a middleware tallies requests answered by
  each replica; surfaced in /healthz, /api/info and the admin replica
  table. Under load-balancing the counts diverge, and a rolling update
  resets them one container at a time — the rollout made visible.
- Rolling-update wave in the Chaos lab: bounces replicas one at a time
  (via the crash endpoint) while polling the replica table, so a training
  session can watch each replica reset and rejoin — what a rolling update
  looks like from the inside.
- Admin console: reactions tile, per-replica Requests column, reaction
  tallies in moderation, and a wall-theme setting.
- Favicon + /favicon.ico redirect.

Tests cover reactions (idempotency, tallies, whitelist) and the theme
setting round-trip.

Akili-Task: tsk_xgg6sfuzkzymyc6gstr4
Akili-Agent: goldfish

admin.go

@@ -93,6 +93,7 @@ func (h *Handler) AdminOverview(c *okapi.Context) error {
 			"healthy":    !h.unhealthy.Load(),
 			"started_at": h.started.UTC().Format(time.RFC3339),
 			"uptime_s":   int64(time.Since(h.started).Seconds()),
+			"requests":   h.requests.Load(),
 			"go":         runtime.Version(),
 			"cpus":       runtime.NumCPU(),
 			"goroutines": runtime.NumGoroutine(),
@@ -167,6 +168,14 @@ func (h *Handler) AdminSaveSettings(c *okapi.Context) error {
 	if len(st.Banner) > maxBannerLen {
 		return c.JSON(http.StatusBadRequest, okapi.M{"error": "banner is too long (max 200 characters)"})
 	}
+	switch st.Theme {
+	case "light", "dark":
+		// forced appearance for every visitor
+	case "", "system":
+		st.Theme = "" // follow each visitor's preference
+	default:
+		return c.JSON(http.StatusBadRequest, okapi.M{"error": "theme must be system, light or dark"})
+	}
 	if err := h.store.SaveSettings(c.Request().Context(), st); err != nil {
 		return c.JSON(http.StatusInternalServerError, okapi.M{"error": "could not save settings"})
 	}