The /api/query cost guardrail (next phase) needs the same singleton
QuotaTracker so its daily-byte and concurrent-slot caps accumulate
across both /api/v2/scan and /api/query BQ-touching paths.
Move `_build_quota_tracker`, `_quota_singleton`, and `_quota_init_lock`
from app/api/v2_scan.py to app/api/v2_quota.py (the natural home; the
factory uses QuotaTracker which already lives there). Re-export the
function from v2_scan.py so the 7 test sites at tests/test_v2_scan.py
(lines 77, 118, 143, 160, 186, 208, 250) keep working without edits.
Crucially do NOT re-export `_quota_singleton` from v2_scan.py — Python
`from X import var` copies the binding at import time, so a re-exported
singleton would freeze at the initial None and never observe the
in-place mutation done inside `_build_quota_tracker()`. Re-export only
the function (which always reads the live module-global through `global`).
Mechanical refactor; no behavior change. 30 quota-related tests pass.
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