feat(backend): add LLM-powered feed enhancement

Add enhancement harness that fills feed item slots and
generates synthetic items via OpenRouter.

- LLM client with 30s timeout, reusable SDK instance
- Prompt builder with mini calendar and week overview
- arktype schema validation + JSON Schema for structured output
- Pure merge function with clock injection
- Defensive fallback in feed endpoint on enhancement failure
- Skips LLM call when no unfilled slots or no API key

Co-authored-by: Ona <no-reply@ona.com>

apps/aris-backend/.env.example

@@ -7,6 +7,11 @@ BETTER_AUTH_SECRET=
 # Base URL of the backend
 BETTER_AUTH_URL=http://localhost:3000
 
+# OpenRouter (LLM feed enhancement)
+OPENROUTER_API_KEY=
+# Optional: override the default model (default: openai/gpt-4.1-mini)
+# OPENROUTER_MODEL=openai/gpt-4.1-mini
+
 # Apple WeatherKit credentials
 WEATHERKIT_PRIVATE_KEY=
 WEATHERKIT_KEY_ID=