feat: add retry logic for transient failures

- @with_retry decorator and retry_async() function
- Exponential backoff with jitter
- Retries on: timeout, connection errors, HTTP 429/5xx
- Web search tool now retries on network failures
- Configurable via RETRY_MAX_ATTEMPTS, RETRY_BASE_DELAY, RETRY_MAX_DELAY
- 29 new tests (205 total passing)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

webber-api/src/shared/config.py

@@ -90,6 +90,11 @@ class Settings(BaseSettings):
     summarization_target_tokens: int = 500  # Target summary size
     keep_recent_messages: int = 6  # Messages to keep unsummarized (3 turns)
 
+    # Retry logic
+    retry_max_attempts: int = 3  # Max retry attempts for transient failures
+    retry_base_delay: float = 1.0  # Base delay in seconds
+    retry_max_delay: float = 30.0  # Maximum delay in seconds
+
     model_config = SettingsConfigDict(
         env_file=".env",
         case_sensitive=False,