refactor(metering): harden page_count guard per review

Round-1 review follow-ups (PR #879):
- Gate pages_embedded on `page_count and page_count > 0` so a malformed
  negative count meters as "no pages" rather than emitting a negative
  billing row (matches the documented call-site intent).
- Exclude bool at the call-site narrowing (`isinstance(int) and not
  isinstance(bool)`) — bool is an int subclass, so a stray page_count=True
  would otherwise record pages=1.
- Document chunk_count's role (empty-batch no-op guard) and the
  intentional tokens-before-pages ordering in the docstring/comments.
- Add test_negative_pages_skips_pages.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/unit/test_processor_metering.py

@@ -96,6 +96,26 @@ async def test_zero_pages_skips_pages(store_spy):
     assert by_metric == {"tokens_embedded": 99}
 
 
+@pytest.mark.unit
+async def test_negative_pages_skips_pages(store_spy):
+    """A malformed negative page_count meters as 'no pages' (tokens only)."""
+    await processor.record_indexing_usage(
+        enabled=True,
+        provider="mistral",
+        model="mistral-embed",
+        doc_type="file",
+        user_id="alice",
+        chunk_count=4,
+        token_count=99,
+        total_chars=1000,
+        page_count=-1,
+    )
+
+    calls = store_spy.record_usage_event.await_args_list
+    by_metric = {c.kwargs["metric"]: c.kwargs["value"] for c in calls}
+    assert by_metric == {"tokens_embedded": 99}
+
+
 @pytest.mark.unit
 async def test_disabled_is_noop(store_spy):
     """Flag off → no store access, no events."""