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/migrations/000002_relation.up.sql

@@ -0,0 +1,13 @@
+CREATE TABLE IF NOT EXISTS relation_insights (
+  id uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  user_id uuid NOT NULL REFERENCES users(id),
+  profile_a_id uuid NOT NULL REFERENCES profiles(id),
+  profile_b_id uuid NOT NULL REFERENCES profiles(id),
+  summary jsonb NOT NULL DEFAULT '{}',
+  report_id uuid NULL REFERENCES growth_reports(id),
+  created_at timestamptz NOT NULL DEFAULT now(),
+  updated_at timestamptz NOT NULL DEFAULT now(),
+  deleted_at timestamptz NULL
+);
+CREATE INDEX IF NOT EXISTS idx_relation_insights_user_id ON relation_insights(user_id);
+CREATE INDEX IF NOT EXISTS idx_relation_insights_profiles ON relation_insights(profile_a_id, profile_b_id);