feat(backend): bootstrap Go module, config, and data plane

Introduce configuration validation, Postgres migrations, Redis/storage adapters, and the seed sample video.

backend/internal/observability/observability_test.go

@@ -0,0 +1,19 @@
+package observability
+
+import "testing"
+
+func TestSetupTracingNoop(t *testing.T) {
+	shutdown, err := SetupTracing(t.Context(), Config{ServiceName: "test", OTLPEndpoint: ""})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if shutdown == nil {
+		t.Fatal("expected shutdown func")
+	}
+	if err := shutdown(t.Context()); err != nil {
+		t.Fatal(err)
+	}
+	ctx, span := StartSpan(t.Context(), "test.span")
+	_ = ctx
+	span.End()
+}