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_config.py

@@ -60,7 +60,13 @@ class TestSessionLimiterConfig:
     """MAX_CONVERSATIONS_PER_RUN and REQUEST_DELAY parsing in load_config."""
 
     def _load(self, monkeypatch, tmp_path, **env):
+        from src import config as config_module
         from src.config import load_config
+        # load_config() calls load_dotenv(override=False), which re-populates
+        # any variable this test just deleted from the developer's real .env —
+        # so test_defaults only saw defaults on a machine without one. Stub it:
+        # these tests are about parsing the environment, not discovering .env.
+        monkeypatch.setattr(config_module, "load_dotenv", lambda *a, **k: False)
         monkeypatch.setenv("EXPORT_DIR", str(tmp_path / "exports"))
         monkeypatch.setenv("CACHE_DIR", str(tmp_path / "cache"))
         for key in (