Closes reviewer suggestion (deleg_5dd358e3): adds coverage for the factor
readiness endpoint (n_points / learnable / last_value / ordering) and the
min_points 400 validation. FACTOR_HISTORY_DIR is now configurable via app
config so tests (and deploy) can point the history store at a chosen path
instead of a hardcoded data dir. 236 tests pass.
b362cc35bf