Bug/restore full weekends

[Vhivi]

Objective

Enforce a minimum number of full free weekends per planning horizon and make backend tests deterministic with the new solver constraint.

Breaking Change Notice

No API change.
This update adds a stricter hard solver constraint and can make some previously feasible schedules infeasible when min_free_weekends_per_horizon is too high for the selected horizon.

Scope

• Add hard constraint enforce_min_free_weekends_per_horizon to solver hard constraints.
• Wire solver.min_free_weekends_per_horizon through solver context/engine.
• Keep weekend logic consistent by removing redundant weekend-balancing soft objective coupling.
• Add targeted tests for the new behavior:
  • feasible case
  • explicit infeasible case
• Stabilize legacy backend tests by isolating them from local runtime config and setting test-specific solver defaults (min_free_weekends_per_horizon=0 where not under test).
• Make modularization tests use a deterministic dedicated runtime config.

Validation

• backend/.venv/Scripts/python.exe -m pytest -q backend/tests -q

Results

• Backend test suite passes globally: 82/82 (100%).
• New weekend minimum rule is covered by dedicated tests.
• Legacy tests no longer depend on local backend/config.json side effects.

Risks and Mitigations

• Risk: stricter weekend constraint may increase infeasible schedules on short/tight horizons.
• Mitigation: parameter is configurable; infeasible settings are surfaced clearly; tests now explicitly control solver configuration.