Add OpenRouter AI provider

## Summary
Implementation plan includes OpenRouter as an AI provider option. Registry only supports nvidia, ollama, and mock.

## Tasks
- [ ] Add `apps/api/src/services/ai/openrouter.ts`
- [ ] Register in `ai/registry.ts` when `AI_PROVIDER=openrouter`
- [ ] Document `OPENROUTER_API_KEY` in `.env.example`
- [ ] Implement `generateFullDeck` compatible with existing pipeline

## Files
- `apps/api/src/services/ai/registry.ts`
- `apps/api/src/services/ai/nvidia.ts` (reference)
- `.env.example`

Start with apps/api/src/services/ai/nvidia.ts as the reference implementation, then add apps/api/src/services/ai/openrouter.ts with a compatible generateFullDeck. Wire it into apps/api/src/services/ai/registry.ts for AI_PROVIDER=openrouter and document OPENROUTER_API_KEY in .env.example. Done means the existing AI pipeline can select the OpenRouter provider.

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