feat: quarantine new releases to prevent supply chain attacks

Add per-remote quarantine support: when quarantine_new=true and quarantine_days=N,
immutable artifacts published within the last N days are blocked with 404 until
the quarantine window expires.

- ConfigManager.get_quarantine_config() reads quarantine_new/quarantine_days
- RedisCache.store/get_artifact_published() persist Last-Modified per artifact
- proxy._check_quarantine() enforces the window; fails open when date is unknown
- proxy._fetch_last_modified() HEAD-requests upstream to discover publish date
- Docker proxy route wires quarantine checks on both cache-hit and cache-miss
- remotes.yaml: quarantine_new/quarantine_days added to pypi example (3-day window)
- README: documents quarantine configuration

tests/conftest.py

@@ -98,6 +98,24 @@ TEST_REMOTES = {
             "immutable_patterns": [r"\.tgz$"],
             "cache": {"immutable_ttl": 0, "mutable_ttl": 3600},
         },
+        "quarantine-test": {
+            "base_url": "https://releases.example.com",
+            "type": "remote",
+            "package": "generic",
+            "immutable_patterns": [r".*\.tar\.gz$"],
+            "quarantine_new": True,
+            "quarantine_days": 3,
+            "cache": {"immutable_ttl": 0, "mutable_ttl": 0},
+        },
+        "quarantine-disabled": {
+            "base_url": "https://releases.example.com",
+            "type": "remote",
+            "package": "generic",
+            "immutable_patterns": [r".*\.tar\.gz$"],
+            "quarantine_new": False,
+            "quarantine_days": 3,
+            "cache": {"immutable_ttl": 0, "mutable_ttl": 0},
+        },
     }
 }