feat(ai-gateway): discount Gemini 3.8 Flash pricing

[chrarnoldus]

Summary

• set non-fallback custom pricing for google/gemini-3.8-flash using OpenRouter's 50%-discounted standard endpoint rates
• mark the promotional pricing as available through the end of 2026
• cover displayed rates and cost recalculation when upstream reports a market cost

Pricing source: https://openrouter.ai/api/v1/models/google/gemini-3.8-flash/endpoints

Testing

• Not run locally; left to CI as requested

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (2 files)

• apps/web/src/lib/ai-gateway/custom-pricing.ts
• apps/web/src/lib/ai-gateway/custom-pricing.test.ts

---

_{Reviewed by grok-4.6 · Input: 110.3K · Output: 12.5K · Cached: 986.1K}

_{Review guidance: REVIEW.md from base branch main}