Largest routes/ slice yet — drives.py (8 endpoints) and burnin.py
(4 endpoints). Drives helpers live in _drives_helpers.py so the
dashboard SSE handler in routes/__init__.py and mailer.py can both
keep using them via re-export.
routes/__init__.py shrinks from 815 → 163 LoC; only the dashboard /
and /sse/drives stream remain there. Routes split is now functionally
complete: 12 files, ~1800 LoC distributed by feature.
