feat: add a channel as a logging channel

Closes #1, fixes #10

migrations/20231122220824_guild_log_channels.sql

@@ -1,6 +1,13 @@
 -- Add migration script here
+
+-- Discord IDs are kept as INTEGERs and not unsigned INTEGERs despite
+-- their Rust type being u64. In order to properly manage them, you'll
+-- need to cast any u64 to i64 with `as i64` before writing them to
+-- the database, and cast any i64 to u64 with `as u64` when reading
+-- them from the database. This operation is noop in Rust and should
+-- therefore not cost a single CPU cycle.
 CREATE TABLE IF NOT EXISTS guild_log_channels (
-       guild_id TEXT PRIMARY KEY,
-       channel_id TEXT NOT NULL,
+       guild_id INTEGER PRIMARY KEY,
+       channel_id INTEGER NOT NULL,
        UNIQUE(guild_id, channel_id)
 );