feat: Add household axes for MTR and earnings variation

[SakshiKekre]

Summary

• Adds optional axes parameter to POST /household/calculate for sweeping a person's input variable across a range (e.g. employment_income from 0 to 200k in 401 steps)
• Returns all output variables as 401-element arrays instead of scalars, enabling MTR charts and earnings variation charts
• Removes the frontend's dependency on the v1 API's calculate-full endpoint for variation calculations
• Axes support added to both local calculation path and Modal functions (UK + US)

Changes

• New: src/policyengine_api/utils/axes.py — validate, expand DataFrames, reshape output
• New: tests/test_axes.py — 18 unit tests for axes logic
• Modified: src/policyengine_api/api/household.py — AxisSpec model, axes field on request, validation, expansion/reshape threading
• Modified: src/policyengine_api/modal_app.py — axes param on simulate_household_uk and simulate_household_us

Test plan

• Unit tests for expand/reshape/validate logic (18 tests passing)
• Integration test: POST to /household/calculate with axes, poll, verify 401-element arrays
• Smoke test: Point alpha FE householdVariation.ts at v2 endpoint, verify MTR + earnings variation charts render