- Add passkey prompt after magic link login (/?add-passkey=1)
- addPasskeyStart/addPasskeyFinish for existing users
- Dev mode: magic link auto-redirects (returns devLink in response)
- Fix username validation: strip invalid chars instead of HTML pattern
- Separate API routes from page routes for JSON fetch compatibility
All 10 Group 7 tasks complete.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>