improve IRC channel continuity and formatting

internal/storage/migrations/006_irc_channel_state.sql

@@ -0,0 +1,15 @@
+-- Migration 006: Remember the channels open for each saved IRC profile.
+
+CREATE TABLE IF NOT EXISTS irc_channel_state (
+    profile_id TEXT NOT NULL,
+    room_name TEXT NOT NULL,
+    position INTEGER NOT NULL DEFAULT 0,
+    updated_at TEXT NOT NULL,
+    PRIMARY KEY (profile_id, room_name)
+);
+
+CREATE INDEX IF NOT EXISTS idx_irc_channel_state_profile_position
+    ON irc_channel_state (profile_id, position);
+
+INSERT INTO schema_version (version, applied_at)
+VALUES (6, datetime('now'));