feat: add check_mutable_updates flag for conditional upstream revalidation

When check_mutable_updates: true is set on a remote, expired user-defined
mutable files are revalidated before re-downloading:

- On expiry a conditional HEAD is sent with If-None-Match / If-Modified-Since
- 304 Not Modified: TTL is refreshed in Redis, S3 cache is untouched
- 200 / no conditional support: cache is invalidated and file re-downloaded
- Network error: safe fallback — assume changed, re-download

ETag and Last-Modified from upstream responses are stored in Redis under
mutable:meta:<remote>:<hash> (no expiry, cleaned up on re-download or
cache flush). The flag only applies to user-configured mutable_patterns;
built-in package-type defaults (APKINDEX, repomd.xml, Docker manifests)
are always re-fetched unconditionally.

cache/flush also clears mutable:meta:* keys alongside index:* keys.

tests/test_config.py

@@ -208,6 +208,39 @@ class TestGetImmutablePatterns:
         assert cfg.get_immutable_patterns("r", "/unknown/path") == [r".*\.tar\.gz$"]
 
 
+# ---------------------------------------------------------------------------
+# get_user_mutable_patterns
+# ---------------------------------------------------------------------------
+
+
+class TestGetUserMutablePatterns:
+    def test_returns_only_user_patterns(self, make_config):
+        cfg = make_config(
+            {
+                "r": {
+                    "type": "remote",
+                    "package": "alpine",
+                    "base_url": "https://x.com",
+                    "mutable_patterns": [r"custom\.json$"],
+                }
+            }
+        )
+        assert cfg.get_user_mutable_patterns("r") == [r"custom\.json$"]
+
+    def test_excludes_package_defaults(self, make_config):
+        # Package defaults (APKINDEX etc.) must NOT appear here
+        cfg = make_config({"r": {"type": "remote", "package": "alpine", "base_url": "https://x.com"}})
+        assert cfg.get_user_mutable_patterns("r") == []
+
+    def test_returns_empty_for_missing_remote(self, make_config):
+        cfg = make_config({})
+        assert cfg.get_user_mutable_patterns("nonexistent") == []
+
+    def test_returns_empty_when_key_absent(self, make_config):
+        cfg = make_config({"r": {"type": "remote", "package": "generic", "base_url": "https://x.com"}})
+        assert cfg.get_user_mutable_patterns("r") == []
+
+
 # ---------------------------------------------------------------------------
 # get_cache_config
 # ---------------------------------------------------------------------------