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/model/relation.go

@@ -0,0 +1,19 @@
+package model
+
+import (
+	"encoding/json"
+	"time"
+
+	"github.com/google/uuid"
+)
+
+// RelationInsight is a dual-profile understanding record.
+type RelationInsight struct {
+	ID         uuid.UUID       `json:"id"`
+	UserID     uuid.UUID       `json:"user_id"`
+	ProfileAID uuid.UUID       `json:"profile_a_id"`
+	ProfileBID uuid.UUID       `json:"profile_b_id"`
+	Summary    json.RawMessage `json:"summary"`
+	ReportID   *uuid.UUID      `json:"report_id,omitempty"`
+	CreatedAt  time.Time       `json:"created_at"`
+}