Refactor/solver modularization v1

[Vhivi]

Summary

Modularize the backend solver architecture and fix cross-year/weekend leave handling so scheduling logic is more maintainable and leave behavior is consistent between backend and frontend.

Changes

• Extracted solver logic from backend/app.py into a modular solver package:
  • context container,
  • constraint registry,
  • grouped constraints (hard, soft, mixed),
  • objective builder,
  • dedicated solver engine orchestration.
• Kept API contract stable by preserving generate_planning(...) facade behavior in routes.
• Added modular solver regression coverage for registry wiring and key hard constraints.
• Added planning-start-date propagation (planning_start_date) from route to solver engine.
• Built deterministic full-date mapping for planning day labels in solver context.
• Updated leave blocking logic to compare full resolved dates instead of dd-mm-only matching.
• Updated backend leave helper to support reference-date-based year-safe resolution.
• Updated PlanningTable to resolve day labels from planningStartDate and match leave/unavailable/training using full config dates (dd-mm-YYYY).
• Ensured weekend days included in leave periods are rendered as Con..
• Updated project docs/changelog for modular solver architecture and date-handling rules.

Tests

• Backend modularization tests added (test_solver_modularization.py).
• Backend regressions added for:
  • leave periods from another year not affecting target planning year,
  • leave helper weekend/reference-year behavior.
• Frontend regressions added for:
  • no false leave match when dd-mm matches but year differs,
  • weekend leave rendering as Con. when weekend dates are inside leave range.
• Full validation:
  • python -m pytest -q => 73 passed
  • npm test -- --runInBand => 2 passed suites, 7 passed tests

Why

This reduces solver monolith complexity and makes constraints easier to evolve safely, while fixing real leave-date correctness issues (year collisions and weekend rendering) in a way that keeps backend and frontend date semantics aligned.