Reasoning-model UX: better empty-response error, higher default budget, live link

- When a model streams reasoning but no story text, return a specific,
  actionable error (raise Max output tokens / set a reasoning cap / use a
  non-reasoning model) instead of the opaque "empty response".
- Bump default max_output_tokens 400 -> 800: 400 truncated scenes and
  left reasoning models with no room after thinking. Affects new settings
  rows; existing users keep their value.
- README: add a "Try it live" link to the Render deployment.

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

backend/app/routers/adventures.py

@@ -288,7 +288,17 @@ async def generate_turn(
 
     text = "".join(chunks).strip()
     if not text:
-        yield sse({"type": "error", "detail": "The AI returned an empty response."})
+        # If the model streamed reasoning but no story text, it spent its whole
+        # budget thinking — say so instead of a mysterious "empty response".
+        if reasoning_chunks:
+            detail = (
+                "The model used its entire token budget on reasoning and returned no "
+                'story text. Raise "Max output tokens" in Settings, set a "Reasoning '
+                'max tokens" cap, or switch to a non-reasoning model.'
+            )
+        else:
+            detail = "The AI returned an empty response."
+        yield sse({"type": "error", "detail": detail})
         return
 
     # onOutput