feat(phase8.1): server groups — table, CRUD handler, collapsible channel sections

internal/db/db.go

@@ -479,4 +479,17 @@ DROP TRIGGER IF EXISTS messages_search_trigger ON messages;
 CREATE TRIGGER messages_search_trigger
 BEFORE INSERT OR UPDATE ON messages
 FOR EACH ROW EXECUTE FUNCTION messages_search_update();
+
+-- Server groups (sub-server sections for organizing channels)
+CREATE TABLE IF NOT EXISTS server_groups (
+	id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+	server_id UUID NOT NULL REFERENCES servers(id) ON DELETE CASCADE,
+	name VARCHAR(100) NOT NULL,
+	position INTEGER NOT NULL DEFAULT 0,
+	created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+	UNIQUE (server_id, name)
+);
+CREATE INDEX IF NOT EXISTS idx_server_groups_server ON server_groups(server_id);
+ALTER TABLE channels ADD COLUMN IF NOT EXISTS group_id UUID REFERENCES server_groups(id) ON DELETE SET NULL;
 `
+