chore: consolidate to single migration, remove AI ORM layer

- Rewrote 0001_consolidated_schema.py: dropped all AI/LLM tables (muse_usage_logs, muse_conversations, muse_conversation_messages, muse_message_actions), DAW variation tables (muse_variations, muse_phrases, muse_note_changes), and budget columns from muse_users. Folded 0002 into 0001 so there is exactly one migration file. - Deleted 0002_drop_muse_variation_tables.py (now superfluous). - Rewrote db/models.py: removed UsageLog, Conversation, ConversationMessage, MessageAction ORM classes and all budget-related columns/properties from User. Only User and AccessToken remain. - Removed UsageLog from db/__init__.py exports. - Fixed tests: removed budget_cents / budget_limit_cents from all User() fixture instantiations.