feat: Auto-group & Suggest Combos (Phase 2)

[nguyenlamzx]

Mục tiêu

Mở rộng từ metadata+filter (Phase 1 #731) sang auto-group và suggest combos.

Tính năng

1. Auto-group: tự phân loại combo theo profile khi save (realtime) + batch regroup
2. Suggest Combos: modal preview 4 profiles (Fast/Cheap/Balanced/Reliable)

Scoring algorithm

• Pricing (input/output cost)
• Provider health (testStatus)
• Usage logs (latency/success rate)
• Model name heuristic (flash/mini/opus...)

API mới

• POST /api/combos/regroup
• POST /api/combos/suggestions
• POST /api/combos/apply-suggestion

Files

• src/lib/comboScoring.js (new)
• src/lib/usageDb.js, src/lib/requestDetailsDb.js (extend)
• src/app/api/combos/*/route.js (extend)
• src/app/(dashboard)/dashboard/combos/page.js (UI modal)