Remove unused stats table data from CONUS and Alaska hydroviz endpoints

[cstephen]

Closes #733

Before this PR, min/median/max values were included for every scenario in the CONUS and Alaska stats table data. We ended up using only the median values across the board in the hydroviz webapp. So, this PR removes all min & max data to reduce unnecessary computation and make the data response a bit smaller.

To test, run the hydroviz webapp against this unused_hydroviz_data Data API branch and verify that CONUS and Alaska stats tables are still populating without errors.

[Copilot]

Pull request overview

This PR reduces aggregation computations and response payload size for hydroviz “stats table” outputs by emitting only the single aggregation statistic actually used by the hydroviz UI (per scenario), rather than always computing min/median/max.

Changes:

• CONUS hydroviz: compute and return only one aggregation statistic per scenario (rcp45→min, rcp60→median, rcp85→max).
• Arctic hydroviz: remove min/max from projected stats tables (including water temperature stats), leaving only median.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
routes/conus_hydrology.py	Updates projected CONUS table stats to compute/return a single per-scenario aggregation value.
routes/arctic_hydrology.py	Removes min/max from Arctic projected stats tables (regular + water temperature), returning only median.

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