add entropy enforcement for optional encryption requirement

Shannon entropy check rejects low-entropy content when MIN_ENTROPY > 0.
Encrypted data ~7.5-8.0 bits/byte, plaintext ~4.0-5.0 bits/byte.
Configurable via FLASKPASTE_MIN_ENTROPY environment variable.

tests/test_abuse_prevention.py

@@ -231,3 +231,68 @@ class TestWindowReset:
             is_allowed, count = check_content_hash(content_hash)
             assert is_allowed is True
             assert count == 1  # Counter reset
+
+
+class TestEntropyEnforcement:
+    """Test minimum entropy requirement."""
+
+    @pytest.fixture
+    def entropy_app(self):
+        """Create app with entropy requirement enabled."""
+        app = create_app("testing")
+        app.config["MIN_ENTROPY"] = 6.0  # Require high entropy
+        return app
+
+    @pytest.fixture
+    def entropy_client(self, entropy_app):
+        """Create test client with entropy requirement."""
+        return entropy_app.test_client()
+
+    def test_plaintext_rejected(self, entropy_client):
+        """Plaintext content should be rejected when entropy required."""
+        response = entropy_client.post(
+            "/",
+            data=b"Hello, this is plain English text with low entropy.",
+            content_type="text/plain",
+        )
+        assert response.status_code == 400
+
+        data = response.get_json()
+        assert data["error"] == "Content entropy too low"
+        assert "entropy" in data
+        assert "min_entropy" in data
+        assert "hint" in data
+
+    def test_random_data_accepted(self, entropy_client):
+        """Random/encrypted data should pass entropy check."""
+        import os
+        random_data = os.urandom(512)  # High entropy random bytes
+
+        response = entropy_client.post(
+            "/",
+            data=random_data,
+            content_type="application/octet-stream",
+        )
+        assert response.status_code == 201
+
+    def test_entropy_disabled_by_default(self, client, sample_text):
+        """Entropy check should be disabled by default (MIN_ENTROPY=0)."""
+        # Default testing config has MIN_ENTROPY=0
+        response = client.post(
+            "/",
+            data=sample_text,
+            content_type="text/plain",
+        )
+        assert response.status_code == 201
+
+    def test_repeated_bytes_rejected(self, entropy_client):
+        """Repeated bytes have zero entropy and should be rejected."""
+        response = entropy_client.post(
+            "/",
+            data=b"a" * 1000,
+            content_type="text/plain",
+        )
+        assert response.status_code == 400
+
+        data = response.get_json()
+        assert data["entropy"] == 0.0