docs: update TODO with Next.js 16 breaking change fixes and root cause analysis

TODO.md

@@ -63,3 +63,27 @@
 - All 84 unit tests passing
 - Database migrations applied successfully
 - TypeScript compilation has pre-existing errors unrelated to our changes
+
+### Completed After Commit 1729dac
+
+#### Next.js 16 Breaking Change Fixes
+- [x] Fixed `params.id` usage in all page components (must use `await params`)
+- [x] Fixed `params.id` usage in all API routes (must use `await params`)
+- [x] Updated client components to use `Promise<{ id: string }>` type
+- [x] Added regression tests for Next.js 16 params Promise handling
+- [x] Verified all 100 unit tests pass
+
+#### Files Updated:
+- Player pages: `profile.tsx`, `schedule.tsx`
+- Tournament pages: `page.tsx`, `results.tsx`, `edit.tsx`, `entry.tsx`
+- API routes: `admin/players/[id]/route.ts`, `users/[id]/route.ts`, `users/[id]/role/route.ts`
+- Tournament API routes: `[id]/route.ts`, `[id]/participants/route.ts`, `[id]/games/bulk/route.ts`
+
+#### Root Cause
+Next.js 16 requires `params` to be awaited in both server components and API routes:
+- Before: `const { id } = params`
+- After: `const { id } = await params`
+
+This was not caught by the unit test suite because:
+- Unit tests test individual functions in isolation
+- E2E tests (Playwright) would catch this but weren't run after the upgrade