chore: commit pending phase-3 work and add TEST_ACCOUNTS.md

Includes planning artifacts (03-CONTEXT, 03-DISCUSSION-LOG, 03-02-SUMMARY),
integration test script, MinIO/auth/docker fixes, and local dev account reference.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/api/auth.py

@@ -173,6 +173,7 @@ async def register(
         used_bytes=0,
     )
     session.add(new_user)
+    await session.flush()  # persist User before Quota FK
     session.add(quota)
     await session.commit()
     await session.refresh(new_user)