storage: page_cache PNGs are written non-atomically and raced by concurrent GETs

[LevMuchnik]

Found during the /plan-eng-review of #45.

render_page (backend/storage.py:79-82) checks the cache at :64 for the read, then unconditionally writes at :81-82 with a plain open(..., "wb"):

• Non-atomic. A reader can observe a half-written PNG under its final name.
• Raced. Two concurrent GETs of the same page both miss the cache and both write the same path.

#45 makes save_original / save_converted / save_filed / save_scanner_test_frame atomic via a new backend/atomic.py, and excludes page_cache from backups entirely — so this stopped being a backup bug. It is still a live serving bug: the user sees a broken image.

Fix: route the cache write through backend/atomic.py's atomic_write_bytes once #45 lands. Should be a two-line change at that point.

Related: #45, #46.