Retry RescueGroups API requests with backoff

A transient ReadTimeout from api.rescuegroups.org crashed the entire run
(it's the only non-debug source), since main.run() only catches ValueError.

Mount an HTTPAdapter with urllib3 Retry on the request session so transient
timeouts and 5xx/429 responses are retried with exponential backoff before
giving up. POST is opted into allowed_methods (not retried by default), and
raise_on_status=False preserves the existing raise_for_status() error path.

Fixes #116

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: MilesMorel

adoption_sources/rescue_groups.py

@@ -11,6 +11,8 @@
 from typing import Iterator
 
 import requests
+from requests.adapters import HTTPAdapter
+from urllib3.util.retry import Retry
 
 from abstractions import AdoptablePet, PetSource
 from config import CITY_NAME, CITY_STATE, POSTAL_CODE
@@ -21,6 +23,27 @@
 # website; those should never be posted. Add new names here as we encounter them.
 PLACEHOLDER_NAMES: tuple[str, ...] = ("more dogs soon!",)
 
+# The RescueGroups API occasionally times out or returns a transient 5xx. A
+# single hiccup shouldn't fail the whole run, so retry a few times with
+# exponential backoff (0s, 2s, 4s, 8s between attempts).
+RETRY_TOTAL = 4
+RETRY_BACKOFF_FACTOR = 1
+
+
+def _session_with_retries() -> requests.Session:
+    """Build a requests Session that retries transient errors with backoff."""
+    retry = Retry(
+        total=RETRY_TOTAL,
+        backoff_factor=RETRY_BACKOFF_FACTOR,
+        status_forcelist=(429, 500, 502, 503, 504),
+        # We only POST, so POST must be opted in (it isn't retried by default).
+        allowed_methods=frozenset({"POST"}),
+        raise_on_status=False,
+    )
+    session = requests.Session()
+    session.mount("https://", HTTPAdapter(max_retries=retry))
+    return session
+
 
 class SourceRescueGroups(PetSource):
     """
@@ -92,7 +115,8 @@ def fetch_pets(self) -> Iterator[AdoptablePet]:
             f"Fetching {self.species} from RescueGroups within {self.radius_miles} miles of {self.postal_code}"
         )
 
-        response = requests.post(url, json=payload, headers=headers, timeout=30)
+        session = _session_with_retries()
+        response = session.post(url, json=payload, headers=headers, timeout=30)
         response.raise_for_status()
 
         body = response.json()