Add flow-rate-aware zone scheduling

[lnagel]

Summary

• Add per-zone nominal_flow_rate configuration (L/min) for flow-rate-aware scheduling
• Add controller-level optimal_flow_rate_min and optimal_flow_rate_max bounds via a new "Flow Scheduling" options menu
• Introduce core/scheduler.py with apply_flow_constraint() to cap aggregate flow by demoting TURN_ON actions to STAY_OFF when the budget is exceeded (front-loading zones with highest remaining quota)
• Suppress boiler heat request when aggregate flow from active zones is below the configured minimum (latent heat mode)

Test plan

• Unit tests for apply_flow_constraint() covering: unconstrained pass-through, budget capping, starvation prevention, zones without flow rate
• Integration tests for flow minimum gating of heat request
• Config flow tests for the new flow scheduling options step
• Verify diff coverage meets 100% threshold

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 23.37662% with 59 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.46%. Comparing base (5210f04) to head (8114562).
⚠️ Report is 8 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
custom_components/ufh_controller/core/scheduler.py	9.67%	28 Missing ⚠️
custom_components/ufh_controller/config_flow.py	21.87%	23 Missing and 2 partials ⚠️
...ustom_components/ufh_controller/core/controller.py	33.33%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #95      +/-   ##
==========================================
- Coverage   96.61%   93.46%   -3.16%     
==========================================
  Files          20       21       +1     
  Lines        1715     1791      +76     
  Branches      257      281      +24     
==========================================
+ Hits         1657     1674      +17     
- Misses         36       91      +55     
- Partials       22       26       +4     

Files with missing lines	Coverage Δ
custom_components/ufh_controller/const.py	100.00% <100.00%> (ø)
custom_components/ufh_controller/coordinator.py	94.70% <100.00%> (+0.02%)	⬆️
custom_components/ufh_controller/core/zone.py	100.00% <100.00%> (ø)
...ustom_components/ufh_controller/core/controller.py	97.22% <33.33%> (-2.78%)	⬇️
custom_components/ufh_controller/config_flow.py	84.35% <21.87%> (-13.62%)	⬇️
custom_components/ufh_controller/core/scheduler.py	9.67% <9.67%> (ø)