code-review fixes: fsync, entropy, is_dm propagation, tests

- Add os.fsync with Windows-compatible directory flush in pairing store
- Increase pairing code length from 6 -> 8 characters for higher entropy
- Remove SystemExit on empty allowFrom; empty list now defers to pairing
- Update is_allowed docstring to document pairing fallback semantics
- Propagate is_dm to Matrix (direct rooms) and Slack (im channels)
- Slack _is_allowed now checks pairing store for DM allowlist mode
- Fix /pairing revoke to accept optional channel argument
- Move inline import time to module top-level
- Add WebSocket comment explaining is_dm=True assumption
- Add comprehensive tests for store and BaseChannel pairing integration
- Fix existing tests that expected empty allowFrom to hard-exit

Refs #3774

tests/channels/test_channel_plugins.py

@@ -961,8 +961,8 @@ class _StartableChannel(BaseChannel):
 
 
 @pytest.mark.asyncio
-async def test_validate_allow_from_raises_on_empty_list():
-    """_validate_allow_from should raise SystemExit when allow_from is empty list."""
+async def test_validate_allow_from_allows_empty_list():
+    """Empty allow_from is valid now — pairing store handles unapproved senders."""
     fake_config = SimpleNamespace(
         channels=ChannelsConfig(),
         providers=SimpleNamespace(groq=SimpleNamespace(api_key="")),
@@ -973,10 +973,8 @@ async def test_validate_allow_from_raises_on_empty_list():
     mgr.channels = {"test": _ChannelWithAllowFrom(fake_config, None, [])}
     mgr._dispatch_task = None
 
-    with pytest.raises(SystemExit) as exc_info:
-        mgr._validate_allow_from()
-
-    assert "empty allowFrom" in str(exc_info.value)
+    # Should not raise — empty list defers to pairing store
+    mgr._validate_allow_from()
 
 
 @pytest.mark.asyncio
@@ -997,8 +995,8 @@ async def test_validate_allow_from_passes_with_asterisk():
 
 
 @pytest.mark.asyncio
-async def test_validate_allow_from_raises_on_empty_dict_allow_from():
-    """_validate_allow_from should reject empty dict-backed allow_from lists."""
+async def test_validate_allow_from_allows_empty_dict_allow_from():
+    """Empty dict-backed allow_from is valid — pairing store handles approval."""
     fake_config = SimpleNamespace(
         channels=ChannelsConfig(),
         providers=SimpleNamespace(groq=SimpleNamespace(api_key="")),
@@ -1009,10 +1007,7 @@ async def test_validate_allow_from_raises_on_empty_dict_allow_from():
     mgr.channels = {"test": _ChannelWithAllowFrom({"enabled": True}, None, [])}
     mgr._dispatch_task = None
 
-    with pytest.raises(SystemExit) as exc_info:
-        mgr._validate_allow_from()
-
-    assert "empty allowFrom" in str(exc_info.value)
+    mgr._validate_allow_from()
 
 
 @pytest.mark.asyncio