feat: formula-based ranking with optional AI, configurable score/pass-rate weights

Add scoreWeight and passRateWeight (0-10) to evaluation config for
configurable composite score formula. When ranking criteria text is
empty, triggerAutoRank uses pure formula ranking (no LLM calls).
When criteria text is present, AI-assisted ranking runs as before.

- Add FORMULA to RankingMode enum with migration
- Extract fetchCategoryProjects helper, add formulaRank service
- Update computeCompositeScore to accept configurable weights
- Add score/pass-rate weight sliders to ranking dashboard UI
- Mode-aware button labels (Calculator/formula vs Sparkles/AI)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

prisma/schema.prisma

@@ -1425,6 +1425,7 @@ enum RankingMode {
   PREVIEW   // Parsed rules shown to admin (not yet applied)
   CONFIRMED // Admin confirmed rules, ranking applied
   QUICK     // Quick-rank: parse + apply without preview
+  FORMULA   // Formula-only: no LLM, pure math ranking
 }
 
 enum RankingSnapshotStatus {