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

handlers.go

@@ -28,6 +28,9 @@ type Handler struct {
 
 	unhealthy atomic.Bool
 	chaos     chaosState
+	// requests counts HTTP requests answered by this replica since boot. A
+	// rolling update resets it one container at a time; see countRequests.
+	requests atomic.Int64
 }
 
 // serverTime is the shape of the live clock payload (v2 feature). It carries
@@ -71,12 +74,13 @@ func (h *Handler) Health(c *okapi.Context) error {
 		})
 	}
 	return c.OK(okapi.M{
-		"status":  "ok",
-		"db":      "up",
-		"redis":   h.broker.RedisStatus(c.Request().Context()),
-		"app":     h.appName,
-		"version": h.version,
-		"host":    h.host,
+		"status":   "ok",
+		"db":       "up",
+		"redis":    h.broker.RedisStatus(c.Request().Context()),
+		"app":      h.appName,
+		"version":  h.version,
+		"host":     h.host,
+		"requests": h.requests.Load(),
 	})
 }
 
@@ -85,13 +89,15 @@ func (h *Handler) Health(c *okapi.Context) error {
 // are currently connected.
 func (h *Handler) Info(c *okapi.Context) error {
 	return c.OK(okapi.M{
-		"app":      h.appName,
-		"version":  h.version,
-		"online":   h.broker.Online(),
-		"host":     h.host,
-		"broker":   h.broker.Mode(),
-		"replicas": h.broker.Replicas(),
-		"admin":    h.auth.Enabled(),
+		"app":       h.appName,
+		"version":   h.version,
+		"online":    h.broker.Online(),
+		"host":      h.host,
+		"broker":    h.broker.Mode(),
+		"replicas":  h.broker.Replicas(),
+		"requests":  h.requests.Load(),
+		"admin":     h.auth.Enabled(),
+		"reactions": ReactionEmojis(),
 	})
 }