feat: add RelationInsight API and wire H5 relation/profile pages

Second P1 growth engine: compare two profiles, gate full tips behind
deep-access mock payment, and list personal archives on /profile.

Co-authored-by: Cursor <cursoragent@cursor.com>

apps/api/internal/relation/engine_test.go

@@ -0,0 +1,22 @@
+package relation
+
+import (
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestBuild(t *testing.T) {
+	a := time.Date(1990, 1, 15, 0, 0, 0, 0, time.UTC)
+	b := time.Date(1992, 6, 8, 0, 0, 0, 0, time.UTC)
+	out := Build(a, b, "我", "TA")
+	if out.Summary["diff_one_liner"] == nil {
+		t.Fatal("missing diff")
+	}
+	blob := str(out.Summary["diff_one_liner"]) + str(out.Detail["title"])
+	for _, bad := range []string{"合盘", "运势", "吉凶", "合婚"} {
+		if strings.Contains(blob, bad) {
+			t.Fatalf("forbidden %q", bad)
+		}
+	}
+}