SFMS: Source data for spatial interpolation of weather and FWI system inputs

**As a** User Type/Persona

**I need** Feature/enhancement

**So That** Value, why is this needed, what does the user need to accomplish

**Acceptance Criteria**
- [ ] Determine where inputs for interpolation are
- [ ] Given (Context), When (action carried out), Then (expected outcome)

**Additional Context**
- enter text here
- enter text here

**Definition of Done**
https://github.com/bcgov/wps/wiki/Definition-of-Done

The issue names no files, tests, or entry points; begin by mapping where spatial-interpolation inputs for weather and the FWI system originate. Clarify the missing context and acceptance criteria, then document the source locations and a verifiable outcome.

Written by the indexing model from the issue text.