test: whole-table tag counts become non-system counts

The four remaining run-1895 failures were stale expectations, not
predicate bugs — prune/reset returned the right counts, but these tests
verified no-deletion by counting the ENTIRE tag table (or asserting the
full kind set), which now includes the three seeded hygiene tags that
survive prunes and resets by design. Filter is_system=false with a
pointer to #128 so future system tags cannot re-break them.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

tests/test_api_admin.py

@@ -660,8 +660,10 @@ async def test_reset_content_tagging_apply_requires_confirm_token(client, db):
     for bad in ({"dry_run": False}, {"dry_run": False, "confirm": "deadbeef"}):
         resp = await client.post("/api/admin/tags/reset-content", json=bad)
         assert resp.status_code == 400
+    # Non-system only: the seeded hygiene tags (#128) survive a reset by
+    # design, so they'd inflate a whole-table count.
     remaining = (await db.execute(
-        select(func.count()).select_from(Tag)
+        select(func.count()).select_from(Tag).where(Tag.is_system.is_(False))
     )).scalar_one()
     assert remaining == 1