Fix OpenAI 429 rate limiting with exponential backoff

The Reddit search uses OpenAI's Responses API with web_search, which
frequently returns 429 rate limit errors. The previous retry logic used
linear backoff (1s, 2s, 3s) which is too aggressive for OpenAI's rate
limiter (often needs 10-60s waits).

Changes:
- Increase max retries from 3 to 5
- Switch from linear to exponential backoff (2s, 5s, 9s, 17s, 33s)
- Parse and respect Retry-After header from OpenAI 429 responses
- Fall back to cheaper models (gpt-4.1 → gpt-4o) on 429s, not just
  on 400/403 access errors
- Remove gpt-4o-mini from fallback chain — it doesn't support
  web_search with the filters parameter

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

scripts/lib/openai_reddit.py

@@ -8,7 +8,8 @@ from typing import Any, Dict, List, Optional
 from . import http
 
 # Fallback models when the selected model isn't accessible (e.g., org not verified for GPT-5)
-MODEL_FALLBACK_ORDER = ["gpt-4.1", "gpt-4o", "gpt-4o-mini"]
+# Note: gpt-4o-mini does NOT support web_search with filters param, so exclude it
+MODEL_FALLBACK_ORDER = ["gpt-4.1", "gpt-4o"]
 
 
 def _log_error(msg: str):
@@ -188,6 +189,9 @@ def search_reddit(
             if _is_model_access_error(e):
                 _log_info(f"Model {current_model} not accessible, trying fallback...")
                 continue
+            if e.status_code == 429:
+                _log_info(f"Rate limited on {current_model}, trying fallback model...")
+                continue
             # Non-access error, don't retry with different model
             raise