fix(billing): apply service tier multipliers to channel pricing

[AixLau]

Summary

• treat flat and interval channel prices as Standard tier prices only
• apply existing priority 2x and flex 0.5x multipliers to final channel pricing
• add regression coverage for exact channel component prices and tier ratios

Test plan

• GOMAXPROCS=2 go test -p=1 -tags=unit ./internal/service -count=1
• GOMAXPROCS=2 go test -p=1 ./internal/service -run "^$"

[github-actions]

All contributors have signed the CLA. ✅
_{Posted by the CLA Assistant Lite bot.}

[AixLau]

I have read the CLA Document and I hereby sign the CLA