refactor(quota): #160 relocate _build_quota_tracker to v2_quota.py

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.

app/api/v2_quota.py

@@ -118,3 +118,45 @@ class QuotaTracker:
     def bytes_used_today(self, user: str) -> int:
         with self._lock:
             return self._ensure_bucket(user)["bytes"]
+
+
+# Module-level singleton (process-local quota state per spec §3.8). FastAPI
+# dispatches sync handlers via a thread pool, so two concurrent first-time
+# requests can both observe `_quota_singleton is None` and each construct a
+# separate tracker; the second assignment wins and the first reference leaks
+# split-brain quota state. Guard with an init lock + double-check.
+#
+# Note: `_quota_singleton` and `_quota_init_lock` are intentionally
+# module-private. Callers MUST go through `_build_quota_tracker()` so the
+# singleton stays single. Re-exporting `_quota_singleton` from another
+# module via `from app.api.v2_quota import _quota_singleton` would copy the
+# initial-None binding at import time and never see subsequent updates —
+# that's a footgun. The function re-export is safe (it always reads the
+# live module-global).
+_quota_init_lock = threading.Lock()
+_quota_singleton: "QuotaTracker | None" = None
+
+
+def _build_quota_tracker() -> QuotaTracker:
+    """Returns or constructs the process-local quota tracker (thread-safe).
+
+    Shared across `/api/v2/scan` (the original caller) and `/api/query`
+    (issue #160 cost guardrail) so the per-user daily byte cap accumulates
+    across both BQ-touching paths.
+    """
+    from app.instance_config import get_value
+    global _quota_singleton
+    if _quota_singleton is not None:
+        return _quota_singleton
+    with _quota_init_lock:
+        if _quota_singleton is None:
+            _quota_singleton = QuotaTracker(
+                max_concurrent_per_user=int(
+                    get_value("api", "scan", "max_concurrent_per_user", default=5) or 5
+                ),
+                max_daily_bytes_per_user=int(
+                    get_value("api", "scan", "max_daily_bytes_per_user", default=53687091200)
+                    or 53687091200
+                ),
+            )
+    return _quota_singleton

app/api/v2_scan.py

@@ -244,28 +244,15 @@ async def scan_estimate_endpoint(
         )
 
 
-# Module-level singleton (process-local quota state per spec §3.8). FastAPI
-# dispatches sync handlers via a thread pool, so two concurrent first-time
-# requests can both observe `_quota_singleton is None` and each construct a
-# separate tracker; the second assignment wins and the first reference leaks
-# split-brain quota state. Guard with an init lock + double-check.
-import threading as _threading
-_quota_init_lock = _threading.Lock()
-_quota_singleton: QuotaTracker | None = None
-
-
-def _build_quota_tracker() -> QuotaTracker:
-    """Returns or constructs the process-local quota tracker (thread-safe)."""
-    global _quota_singleton
-    if _quota_singleton is not None:
-        return _quota_singleton
-    with _quota_init_lock:
-        if _quota_singleton is None:
-            _quota_singleton = QuotaTracker(
-                max_concurrent_per_user=int(get_value("api", "scan", "max_concurrent_per_user", default=5) or 5),
-                max_daily_bytes_per_user=int(get_value("api", "scan", "max_daily_bytes_per_user", default=53687091200) or 53687091200),
-            )
-    return _quota_singleton
+# `_build_quota_tracker` lives in `app.api.v2_quota` so /api/query (issue #160)
+# can share the same singleton without inverting the dep direction
+# (api/query → api/v2/scan would couple a high-level endpoint to a sibling).
+# Re-exported here so existing test sites that call
+# `v2_scan._build_quota_tracker()` (7 in tests/test_v2_scan.py) keep working.
+# Do NOT re-export `_quota_singleton` — `from X import var` copies the
+# binding at import time, so a re-exported singleton would never see the
+# initialized value (#160 review caveat).
+from app.api.v2_quota import _build_quota_tracker  # re-export
 
 
 def _max_result_bytes() -> int: