feat: extensible transforms pipeline for zarr build

[turban]

Summary

• Replaces the hardcoded _UNIT_CONVERSIONS dict and pre_process list with a single transforms pipeline in the dataset YAML
• Each entry is a dotted-path callable (string or {function, params} dict), resolved at runtime the same way ingestion.function works
• Adds src/climate_api/transforms/ with two built-in transforms: convert_units and deaccumulate_era5
• Updates era5_land.yaml to use transforms: for both temperature and precipitation datasets

Closes #79

Usage

transforms:
  - climate_api.transforms.deaccumulate_era5
  - climate_api.transforms.convert_units

External transforms from dhis2eo or any other package can be referenced by dotted path without changes to core code.

Test plan

• uv run pytest tests/test_transforms.py — 12 new tests covering unit conversion, deaccumulation, pipeline execution, and edge cases
• uv run pytest — full suite passes (153 + 12 tests)