Add stream temp endpoints

[Joshdpaul]

Summary

This branch adds two new endpoints to the Arctic Hydrology API for water temperature data:

• /arctic_hydrology/wt_stats/<stream_id> — water temperature statistics (era-level summary stats) for all models. Supports source parameter (gcm_diff_applied_to_cheng default, original_gcm, gcm_diff) and format=csv.
• /arctic_hydrology/wt_modeled_climatology/<stream_id> — daily water temperature climatologies (min/mean/max per day of year) for all models. Supports source parameter (gcm_diff_applied_to_cheng default, original_gcm) and format=csv. gcm_diff is not valid for this endpoint (returns 400), consistent with the streamflow climatology endpoint.

Both endpoints follow the same structure and parameter conventions as the existing stats and modeled_climatology endpoints. PGW models are included under original_gcm and excluded from the Cheng-adjusted and diff sources. CSV output includes full variable comment headers.

Files changed

• routes/arctic_hydrology.py — new route handlers and a new calculate_and_apply_gcm_diffs_to_cheng_wt_climatology helper (additive delta, vs. multiplicative ratio used for streamflow). We use additive here since temperature is not zero bounded like flow.
• csv_functions.py — water temperature CSV metadata blocks for both new endpoints
• templates/documentation/arctic_hydrology.html — documentation for both endpoints, output schema examples, variable reference table, and four new source citations to provide more info about inputs to the stream flow model

Known issues / follow-up needed

• There is no arctic_hydrology/hydroviz endpoint here yet
• We may need to reconsider calling the historical baseline "Cheng". After reading that paper, it seems like the Cheng (2024) data provides the hydroclimate inputs to the Blaskey (2024) hindcast, which actually has the stream flow and stream temp data. XREF: Reconsider the Cheng stream temp baseline arctic_rivers#13 and Reconsider the Cheng stream temp baseline #726

[Copilot]

Pull request overview

This PR adds modeled water temperature support to the existing Arctic Hydrology API patterns by introducing stream-ID endpoints for water temperature statistics and daily climatologies, alongside corresponding CSV metadata and documentation updates.

Changes:

• Added /arctic_hydrology/wt_stats/<stream_id> and /arctic_hydrology/wt_modeled_climatology/<stream_id> route handlers, including a helper to apply additive GCM deltas to the historical baseline for temperature climatologies.
• Extended arctic_hydrology_csv to emit water-temperature-specific CSV metadata blocks for the new endpoints.
• Updated the Arctic Hydrology documentation page to describe the new endpoints, output schemas, variable tables, and additional source citations.

Reviewed changes

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

File	Description
routes/arctic_hydrology.py	Adds new WT endpoints + additive Cheng-adjustment helper; extends metadata packaging call sites for WT climatologies.
csv_functions.py	Adds WT statistics/climatology CSV comment headers and source-note handling.
templates/documentation/arctic_hydrology.html	Documents new WT endpoints, response examples, and supporting citations.

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[cstephen]

I kicked the tires on this to make sure the endpoints work, the optional source parameter works as expected, CSV exports work and have the correct metadata for each endpoint, the documentation looks good, the numbers appear sane, and the code is followable. Approved!