feat: channel key support for +k channels

Add channel_keys dict to NetworkConfig for storing per-channel keys.
Keys are used in KICK rejoin, passed via AUTOJOIN +#channel key syntax,
supported in ADDNETWORK channel_keys= parameter, and propagated through
REHASH. Extract rehash() as reusable async function for SIGHUP reuse.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_config.py

@@ -124,6 +124,28 @@ tls = true
         cfg = load(_write_config(config))
         assert cfg.networks["test"].port == 6697
 
+    def test_channel_keys_parsed(self):
+        config = """\
+[bouncer]
+password = "x"
+
+[proxy]
+
+[networks.test]
+host = "irc.example.com"
+channels = ["#secret", "#public"]
+channel_keys = { "#secret" = "hunter2" }
+"""
+        cfg = load(_write_config(config))
+        net = cfg.networks["test"]
+        assert net.channel_keys == {"#secret": "hunter2"}
+        assert "#secret" in net.channels
+
+    def test_channel_keys_default_empty(self):
+        cfg = load(_write_config(MINIMAL_CONFIG))
+        net = cfg.networks["test"]
+        assert net.channel_keys == {}
+
     def test_operational_defaults(self):
         """Ensure all operational values have sane defaults."""
         cfg = load(_write_config(MINIMAL_CONFIG))