chore: reworked some specs and did small addition to database schema based on gaps in spec, also rechecked the core plan

crates/bds-core/migrations/20260718220000_add_chat_message_token_usage/down.sql

@@ -0,0 +1,3 @@
+-- This file should undo anything in `up.sql`
+ALTER TABLE chat_messages DROP COLUMN token_usage_output;
+ALTER TABLE chat_messages DROP COLUMN token_usage_input;

crates/bds-core/migrations/20260718220000_add_chat_message_token_usage/up.sql

@@ -0,0 +1,3 @@
+-- Your SQL goes here
+ALTER TABLE chat_messages ADD COLUMN token_usage_input INTEGER;
+ALTER TABLE chat_messages ADD COLUMN token_usage_output INTEGER;