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>

config/bouncer.example.toml

@@ -49,6 +49,7 @@ port = 6697
 tls = true
 # nick = "mynick"       # optional: override host-derived nick
 channels = ["#test"]
+# channel_keys = { "#secret" = "hunter2" }  # keys for +k channels
 autojoin = true
 
 # [networks.oftc]