docs: Add README with full backend documentation and smoke test checklist

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

whattotest.txt

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+Refactoring Smoke Test Checklist
+=================================
+
+1.  [x] Start server (npm run dev) — does it boot without import errors?
+2.  [x] Login — check audit log entry exists
+3.  [x] Create a company — check audit log
+4.  [x] Add a note to the company
+5.  [x] Add a reminder to the company
+6.  [x] Assign a user to the company
+7.  [x] Create a project, assign user
+8.  [x] Create a todo with assigned users — check push notification
+9.  [x] Start/stop/pause/resume a timer
+10. [x] Generate a monthly timesheet XLSX — open and verify
+11. [x] Upload a file to a project (check size/type limits)
+12. [x] AI Kurzy: create course, participant, registration