fix(backend): count night shifts in weekly hour cap

[Vhivi]

Objective

Fix weekly worked-hours calculation so night shifts are included in the per-agent weekly cap, then make that cap configurable through solver.max_weekly_hours.

Breaking Change Notice

No API breaking change.

The default weekly cap remains 36h, preserving existing behavior. Schedules that previously exceeded the real weekly worked-hours cap because night shifts were not counted may now correctly become infeasible.

Scope

• Count all configured worked vacations in the weekly hours cap, including Nuit.
• Add configurable solver.max_weekly_hours, expressed in hours, with default 36.
• Wire max_weekly_hours through solver config loading and SolverContext.
• Update config schema and example config.
• Clarify documentation for:
  • strict weekly worked-hours cap
  • paid-hour balance gaps (global_max_gap, period_max_gap)
• Add regression tests for:
  • default 36h cap rejecting 2 Jour + 2 Nuit
  • configured 48h cap accepting the same worked week
  • schema validation for max_weekly_hours

Validation

• backend/.venv/Scripts/python.exe -m pytest backend/tests/test_constraints.py -q
• backend/.venv/Scripts/python.exe -m pytest backend/tests/test_dynamic_solver_config.py backend/tests/test_config_schema.py -q -k "not test_config_json_matches_schema"
• backend/.venv/Scripts/python.exe -m pytest backend/tests -q

Results

• Constraint tests pass: 10/10.
• Dynamic solver config and schema tests pass when excluding local backend/config.json: 11 passed, 1 deselected.
• Full backend suite: 86 passed, 1 failed.

The remaining failure is local-config related and out of scope: backend/config.json contains restriction_types_durations, a key used by another branch but not yet declared in this branch schema.

Risks and Mitigations

• Risk: some schedules that were incorrectly feasible may now be rejected because night hours are counted.
• Mitigation: the cap is configurable via solver.max_weekly_hours; default remains 36h.
• Risk: future half-vacation behavior may require a higher cap such as 42h.
• Mitigation: this PR prepares the configuration hook without introducing half-vacations yet.