fix: increase AI timeout to 120s and limit retries to 1

The 45s timeout was too short for generation tasks with full flow
context in the system prompt. The Anthropic SDK's default 2 retries
caused requests to hang for ~136s before failing. Now: 120s timeout
with max 1 retry = faster failure if it does timeout.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/core/ai_provider.py

@@ -184,6 +184,7 @@ class AnthropicProvider(AIProvider):
         client = anthropic.AsyncAnthropic(
             api_key=self._api_key,
             timeout=self._timeout,
+            max_retries=1,
         )
 
         response = await client.messages.create(