Normalize the fuel_types table

**Describe the task**
Normalize the fuel_types table to allow for unique records for having nullable percent conifer, percent dead fir, and percent grass cure fields.

**Acceptance Criteria**

- [ ] Automatically derive specific criteria from the task description.
- [ ] Ensure the criteria are actionable and testable.
- [ ] Include any edge cases or special conditions mentioned in the description.

**Additional context**

- Add any other context about the task here.
- Or here

The payload names no files or tests. Start by locating the fuel_types table definition and its migration history, then trace how nullable percent conifer, percent dead fir, and percent grass cure fields participate in uniqueness. Done means the table permits the intended unique records and the behavior is covered by actionable, testable criteria.

Written by the indexing model from the issue text.