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/docs/architecture.md

@@ -217,6 +217,9 @@ All settings via environment variables or `.env`:
 | SUMMARIZATION_THRESHOLD | 0.8 | Summarize at N% of max tokens |
 | SUMMARIZATION_TARGET_TOKENS | 500 | Target summary size |
 | KEEP_RECENT_MESSAGES | 6 | Messages to keep unsummarized |
+| RETRY_MAX_ATTEMPTS | 3 | Max retry attempts for transient failures |
+| RETRY_BASE_DELAY | 1.0 | Base delay between retries (seconds) |
+| RETRY_MAX_DELAY | 30.0 | Maximum delay between retries (seconds) |
 
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