Add GEBCO 2025 seafloor bathymetry as a released database layer

[bbest]

Context

ctd-viz needs seafloor depth to draw the bathymetry polygon under transect plots and a raster layer on the cruise map. It currently crops GEBCO bathymetry app-side in apps/ctd-viz/prep_db.R → data/gebco_calcofi.tif.

This is a stopgap — bathymetry should be a first-class layer in the release pipeline so every app/consumer shares one authoritative source.

Current stopgap (done in apps/ctd-viz)

apps/ctd-viz/prep_db.R crops gebco_2025_sub_ice_n90.0_s0.0_w-180.0_e-90.0.tif to the CalCOFI AOI (lon -126..-115, lat 29..36), negates elevation → positive-down depth (m), clamps land to 0, writes data/gebco_calcofi.tif (~3.5 MB, 15 arc-sec).

The prior source (a derived MSENS product) had NA holes in the ocean nearshore, which broke the transect bathymetry polygon — that's what motivated this.

Proposed (this issue)

• Ingest GEBCO 2025 sub-ice bathymetry in workflows/, clipped to the CalCOFI grid extent.
• Decide storage: a raster artifact in the release (COG / PMTiles) vs. depth sampled onto ctd_cast / site rows.
• At minimum: add seafloor_depth_m to ctd_cast (or site) by sampling GEBCO at each station — most consumers just need depth-at-station.
• Update apps/ctd-viz to read released bathymetry; drop the app-side crop in prep_db.R.
• Document source + license (GEBCO 2025 Grid, sub-ice topo/bathy).

Source: GEBCO_2025 Grid sub-ice — https://www.gebco.net/data_and_products/gridded_bathymetry_data/

Related: CalCOFI/apps#40