fix(tests): date the settled snapshot fixture on the CT clock

[westonplatter]

Summary

tests/test_positions_api.py::test_superseded_live_only_row_is_absent_while_a_current_one_is_served
has failed in CI on every run since #219 merged, while passing locally. It is
not flaky and it is not a timing race — it fails on any machine whose local
calendar date is ahead of the Chicago one, which is every UTC machine after
00:00 UTC. CI runs around 02:00 UTC.

Two lines of test-fixture change; src/ is untouched.

Fixes

make_position set as_of_date=date.today(), which reads the process
timezone. is_overlay_superseded compares that date against
ct_date(fetched_at) in America/Chicago. Between 19:00 CT and midnight CT
those two clocks name different days, so the fixture dated its settled snapshot
one day ahead of its own capture — a state the real FlexQuery sync cannot
produce.

The test then reads exactly as designed: with as_of > ct_date(capture), the
live overlay taken at that same instant is superseded, dropped, and the row the
assertion looks for is None.

E       TypeError: 'NoneType' object is not subscriptable
>       assert _row(payload, CON_ES)["source"] == "live"

The fixture now derives the trade date from the capture instant with ct_date
— the same helper production compares against — and accepts an explicit
as_of_date override for tests that need one. The overlay logic was right; the
fixture disagreed with it about which clock names a trade date.

Also adds one guard asserting that invariant directly, so a future
date.today() fails on the reason rather than on a downstream row quietly
going missing.

Verification

TZ=UTC          uv run --group dev --extra mcp pytest   # 246 passed
TZ=Asia/Tokyo   ...                                     # 246 passed
TZ=America/Denver (local)                               # 246 passed

TZ=UTC reproduces the CI failure exactly on main — that is what identified
it. All three timezones pass with this change; before it, the first two fail
and the third does not.

Additional notes

• Tracked as ngv-trader-bjr.
• This is what makes CI readable again on chore(deps): group low-risk frontend bumps (ai, @ai-sdk/react, @types/react) #221 and chore(deps): group low-risk python bumps (alembic, sqlalchemy, langgraph, ruff) #222 (the grouped dependency
  PRs), both of which inherited the red check from main.
• Worth considering separately: src/api/routers/positions.py:167 computes DTE
  with date.today(), the same process-clock assumption in production code.
  Left alone here — it deserves its own change.

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