fix: surface the response body on 4xx so 403s are diagnosable

Media downloads logged "HTTP Error 403:" with no reason. That string is
curl_cffi's raise_for_status() format, "HTTP Error {code}: {reason}", and
HTTP/2 carries no reason phrase — so the message said nothing, and
_make_request threw the response body away. The provider's JSON `detail`
is the only explanation available for a refused asset.

- base._make_request: end non-retryable statuses with a ProviderError
  carrying the body's detail/error/message (redacted, truncated to 300
  chars) instead of a bare raise_for_status().
- media: bucket 403 as `forbidden` in the run summary, separately from
  `download-error` — "the asset is gone" and "we were refused" are
  different problems.
- utils.redact_secrets: match secret key names per word. Exact matching
  let access_token, api_key, and session-token through into logged
  bodies; "keywords"/"monkey"/"tokenizer" stay intact.
- tests/test_config.py: test_defaults depended on the absence of a local
  .env — load_config() calls load_dotenv(override=False), which restored
  the variable the test had just deleted. Stub dotenv discovery.

305 tests pass.

tests/test_providers.py

@@ -1116,3 +1116,69 @@ class TestClaudeDriftCanary:
         p = self._provider([{"uuid": "u1", "name": "N", "updated_at": "z"}],
                            self._detail([]))
         assert DRIFT_ERROR in _sev(p.check_drift())
+
+
+# ---------------------------------------------------------------------------
+# 4xx diagnostics: curl_cffi renders raise_for_status() as
+# "HTTP Error {code}: {reason}", and HTTP/2 has no reason phrase — so a bare
+# 403 logged as "HTTP Error 403:" says nothing. The body carries the cause.
+# ---------------------------------------------------------------------------
+
+
+class TestErrorBodyDiagnostics:
+    class _Resp:
+        ok = False
+        status_code = 403
+        reason = ""
+        headers: dict = {}
+
+        def __init__(self, payload=None, text=""):
+            self._payload = payload
+            self.text = text
+
+        def json(self):
+            if self._payload is None:
+                raise ValueError("not json")
+            return self._payload
+
+    def _provider(self, response):
+        from src.providers.chatgpt import ChatGPTProvider
+        p = ChatGPTProvider.__new__(ChatGPTProvider)
+        p._request_delay = 0
+        p._last_request_at = None
+        p._session = type("S", (), {"request": lambda *a, **k: response})()
+        return p
+
+    def test_detail_field_surfaces_in_error(self):
+        from src.providers.base import ProviderError
+
+        resp = self._Resp(payload={"detail": "File not accessible to this account"})
+        with pytest.raises(ProviderError) as exc:
+            self._provider(resp)._make_request("GET", "https://x/files/f1/download")
+        message = str(exc.value.original)
+        assert "403" in message
+        assert "File not accessible to this account" in message
+
+    def test_non_json_body_excerpted(self):
+        from src.providers.base import ProviderError
+
+        resp = self._Resp(text="<html>Forbidden</html>")
+        with pytest.raises(ProviderError) as exc:
+            self._provider(resp)._make_request("GET", "https://x/files/f1/download")
+        assert "Forbidden" in str(exc.value.original)
+
+    def test_empty_body_says_so_rather_than_nothing(self):
+        from src.providers.base import ProviderError
+
+        resp = self._Resp(text="")
+        with pytest.raises(ProviderError) as exc:
+            self._provider(resp)._make_request("GET", "https://x/files/f1/download")
+        assert "empty response body" in str(exc.value.original)
+
+    def test_secrets_in_error_body_are_redacted(self):
+        from src.providers.base import _describe_error_body
+
+        resp = self._Resp(payload={"error": {"message": "no", "access_token": "sk-abc"}})
+        described = _describe_error_body(resp)
+        assert "sk-abc" not in described
+        assert "[REDACTED]" in described