feat(pitch-admin): backfill first_activity_at for existing investors

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pitch-deck/app/api/admin/migrate/route.ts

@@ -16,6 +16,11 @@ export async function POST(request: NextRequest) {
     `CREATE INDEX IF NOT EXISTS idx_pitch_investors_mask_check
        ON pitch_investors (first_activity_at)
        WHERE first_activity_at IS NOT NULL AND data_masked_at IS NULL`,
+    // 004b — backfill first_activity_at for existing investors from last_login_at
+    `UPDATE pitch_investors
+     SET first_activity_at = last_login_at
+     WHERE last_login_at IS NOT NULL
+       AND first_activity_at IS NULL`,
     `CREATE TABLE IF NOT EXISTS fp_scenarios (
       id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
       name TEXT NOT NULL DEFAULT 'Base Case',