Rename API to /api/water and standardize request/response shapes

[dillonstreator]

Summary

• Rename the lookup endpoints from /api/is-on-water to /api/water so the path is not redundant with the product/domain name.
• Wrap batch POST as { coordinates } / { results } (bare arrays still accepted as input).
• Return RFC 9457 Problem Details (application/problem+json) for errors instead of { msg }.

Test plan

• pnpm test passes
• GET /api/water?lat=20.112682&lon=-37.048647 returns { water, lat, lon }
• POST /api/water with { "coordinates": [...] } returns { "results": [...] }
• Invalid coords return 400 with application/problem+json
• Landing page try-it and docs still hit /api/water
• Swagger at /documentation shows /api/water only