Show a friendly message on 429 instead of raw JSON

Map HTTP 429 in _friendly_http_error: OpenRouter's free-models-per-day cap
gets a "daily limit, resets 00:00 UTC" message; other rate limits get a
generic "too many requests, try again" note. Avoids leaking the raw error
JSON (incl. user_id) to players.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TEpWMjnfPqzZ13nPMoGhs5

backend/app/providers/openai_compatible.py

@@ -267,4 +267,13 @@ class OpenAICompatibleProvider(Provider):
                 f"Endpoint or model not found (HTTP 404). Check the endpoint URL and that "
                 f"model '{self.model}' exists. {detail}"
             )
+        if status == 429:
+            # OpenRouter's shared free tier has a per-day cap; distinguish it
+            # from a short-term burst limit so the message is actionable.
+            if "free-models-per-day" in detail:
+                return (
+                    "The free demo has hit its daily request limit (resets at "
+                    "00:00 UTC). Please try again later."
+                )
+            return "The AI is getting too many requests right now — wait a moment and try again."
         return f"AI endpoint returned HTTP {status}: {detail}"