feat(phase3): threads backend + thread panel UI

internal/db/db.go

@@ -316,6 +316,34 @@ CREATE TABLE IF NOT EXISTS channel_overrides (
 CREATE INDEX IF NOT EXISTS idx_channel_overrides_channel ON channel_overrides(channel_id);
 CREATE INDEX IF NOT EXISTS idx_channel_overrides_target ON channel_overrides(channel_id, target_type, target_id);
 
+-- Threads: reuse channels table by adding parent_channel_id; thread messages are still messages scoped to the thread channel.
+ALTER TABLE channels ADD COLUMN IF NOT EXISTS parent_channel_id UUID REFERENCES channels(id) ON DELETE CASCADE;
+ALTER TABLE channels ADD COLUMN IF NOT EXISTS archived_at TIMESTAMPTZ;
+ALTER TABLE channels ADD COLUMN IF NOT EXISTS auto_archive_duration INTEGER NOT NULL DEFAULT 1440;
+ALTER TABLE channels ADD COLUMN IF NOT EXISTS message_count INTEGER NOT NULL DEFAULT 0;
+ALTER TABLE channels ADD COLUMN IF NOT EXISTS last_message_at TIMESTAMPTZ;
+CREATE INDEX IF NOT EXISTS idx_channels_parent ON channels(parent_channel_id);
+
+-- Forum tags
+CREATE TABLE IF NOT EXISTS forum_tags (
+	id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+	channel_id UUID NOT NULL REFERENCES channels(id) ON DELETE CASCADE,
+	name VARCHAR(64) NOT NULL,
+	emoji VARCHAR(32),
+	color VARCHAR(7),
+	created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+	UNIQUE (channel_id, name)
+);
+
+CREATE INDEX IF NOT EXISTS idx_forum_tags_channel ON forum_tags(channel_id);
+
+-- Thread tag assignments (for forum posts)
+CREATE TABLE IF NOT EXISTS channel_tags (
+	channel_id UUID NOT NULL REFERENCES channels(id) ON DELETE CASCADE,
+	tag_id UUID NOT NULL REFERENCES forum_tags(id) ON DELETE CASCADE,
+	PRIMARY KEY (channel_id, tag_id)
+);
+
 -- Message full-text search
 ALTER TABLE messages ADD COLUMN IF NOT EXISTS search_vector tsvector;
 CREATE INDEX IF NOT EXISTS idx_messages_search ON messages USING GIN(search_vector);