Trade search: hoist loop-invariant field_stats out of the per-candidate band loop

[alhart2015]

In the trade-search route (api_trade_search in src/fantasy_baseball/web/season_routes.py), the analytic-band fill loop recomputes field_stats once per candidate even though it depends only on the loop-invariant config.team_name:

for group in results:                       # ~line 966
    ...
    for cand in group["candidates"]:        # ~line 969
        try:
            ...
            field_stats = projected_standings.field_stats(config.team_name)  # ~line 974
            band = compute_one_for_one_band(..., field_stats=field_stats, ...)

projected_standings.field_stats(config.team_name) returns the same value for every candidate in every group, so it can be hoisted to a single call before the for group in results loop and reused. On a search that returns many candidates this is redundant work on a user-facing request path.

Scope

• Compute field_stats once (right after the if projected_standings is not None: guard, alongside fr) and pass the hoisted value into compute_one_for_one_band.
• Confirm field_stats() is genuinely invariant across candidates (it takes only config.team_name); if it has any per-candidate dependency, this is a no-op — verify before hoisting.
• Guard: tests/test_web/test_trade_search_route.py already exercises the band path.

Context
Pre-existing (not introduced by the #227 type-cleanup); spotted during the loop-review of PR #231 and deliberately left out of scope there since that PR was types-only.