✨ feat(server): wire sampler hot-reload callback (ADR-0023 Phase 3, sub-phase 3.3)

Closes ADR-0023 hot-reload Phase 3 (telemetry sampler). Last sub-phase
of the 3-PR sequence: 3.1 = ReconfigureTracerProvider in pkg/telemetry
(PR #45), 3.2 = sampler callback plumbing in pkg/config (in main via
PR #47 race-merge — see Q-038 in mistral-quirks.md), 3.3 = wire it in
pkg/server.Run (this PR).

Implementation:
- Capture telemetrySetup pointer (was a local) so it outlives initial
  setup and the sampler callback can mutate it.
- After WatchAndApply registration, register a callback that:
  1. Mutates SamplerType/SamplerRatio on the captured Setup
  2. Calls ReconfigureTracerProvider (which builds the new TP, swaps
     the global, drains the old)
  3. Updates s.tracerProvider so graceful shutdown still drains the
     correct provider.
- Callback only registered if telemetrySetup != nil (telemetry was
  successfully initialized at startup; hot-reloading telemetry-on is
  out of scope per ADR-0023).

ADR-0023 status updated: Phase 1+2+3 Implemented. api.v2_enabled
remains explicitly deferred (router refactor needed; separate ADR).

Race detector clean on touched packages. Full BDD suite green.

pkg/server/server.go

@@ -679,10 +679,11 @@ func (s *Server) Router() http.Handler {
 func (s *Server) Run() error {
 	// Initialize OpenTelemetry if enabled
 	var err error
+	var telemetrySetup *telemetry.Setup
 	if s.withOTEL {
 		log.Trace().Msg("Initializing OpenTelemetry tracing")
 
-		telemetrySetup := &telemetry.Setup{
+		telemetrySetup = &telemetry.Setup{
 			ServiceName:  s.config.GetServiceName(),
 			OTLPEndpoint: s.config.GetOTLPEndpoint(),
 			Insecure:     s.config.GetTelemetryInsecure(),
@@ -694,6 +695,7 @@ func (s *Server) Run() error {
 		if s.tracerProvider, err = telemetrySetup.InitializeTracing(context.Background()); err != nil {
 			log.Error().Err(err).Msg("Failed to initialize OpenTelemetry, continuing without tracing")
 			s.withOTEL = false
+			telemetrySetup = nil
 		} else {
 			log.Trace().Msg("OpenTelemetry tracing initialized successfully")
 		}
@@ -714,7 +716,27 @@ func (s *Server) Run() error {
 		s.userService.StartJWTSecretCleanupLoop(rootCtx, s.config.GetJWTSecretCleanupInterval())
 	}
 
-	// Start config hot-reload watcher (ADR-0023 Phase 1: logging.level only).
+	// Wire the sampler hot-reload callback (ADR-0023 Phase 3, sub-phase 3.3).
+	// telemetrySetup is non-nil only when telemetry was successfully initialized
+	// at startup — hot-reloading telemetry-on is out of scope (see ADR-0023).
+	// The callback updates the SamplerType/Ratio on the captured Setup, then
+	// rebuilds the global tracer provider via ReconfigureTracerProvider.
+	if telemetrySetup != nil {
+		s.config.SetSamplerReconfigureCallback(func(ctx context.Context, samplerType string, samplerRatio float64) error {
+			telemetrySetup.SamplerType = samplerType
+			telemetrySetup.SamplerRatio = samplerRatio
+			newTP, rerr := telemetrySetup.ReconfigureTracerProvider(ctx, s.tracerProvider)
+			if rerr != nil {
+				return rerr
+			}
+			if newTP != nil {
+				s.tracerProvider = newTP
+			}
+			return nil
+		})
+	}
+
+	// Start config hot-reload watcher (ADR-0023 Phase 1+2+3).
 	// Stops automatically on rootCtx cancellation.
 	s.config.WatchAndApply(rootCtx)