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/repository/relation_repo.go

@@ -0,0 +1,28 @@
+package repository
+
+import (
+	"context"
+	"encoding/json"
+
+	"github.com/google/uuid"
+	"github.com/jackc/pgx/v5/pgxpool"
+
+	"github.com/yuxingu/digital-psychology/apps/api/internal/model"
+)
+
+// RelationRepo persists relation insights.
+type RelationRepo struct {
+	Pool *pgxpool.Pool
+}
+
+// Create inserts a relation insight row.
+func (r *RelationRepo) Create(ctx context.Context, userID, aID, bID uuid.UUID, summary json.RawMessage, reportID uuid.UUID) (*model.RelationInsight, error) {
+	row := &model.RelationInsight{}
+	err := r.Pool.QueryRow(ctx, `
+		INSERT INTO relation_insights(user_id, profile_a_id, profile_b_id, summary, report_id)
+		VALUES ($1,$2,$3,$4,$5)
+		RETURNING id, user_id, profile_a_id, profile_b_id, summary, report_id, created_at`,
+		userID, aID, bID, summary, reportID,
+	).Scan(&row.ID, &row.UserID, &row.ProfileAID, &row.ProfileBID, &row.Summary, &row.ReportID, &row.CreatedAt)
+	return row, err
+}