Clarify attrs for internals, Pydantic for API boundary only

CLAUDE.md

@@ -101,7 +101,8 @@ Package-specific conventions (attrs patterns, type annotations, testing) are in
 `packages/codeflash-api/` is a ground-up FastAPI rewrite of the Django aiservice from `codeflash-internal/django/aiservice/`. The Django version is the reference implementation — port logic faithfully, don't reimplement from scratch.
 
 Key design decisions:
-- FastAPI with async throughout, Pydantic v2 models at every boundary
+- FastAPI with async throughout
+- Pydantic v2 for request/response schemas only (API boundary). attrs for all internal domain models — same as the rest of the repo
 - No Django ORM — use async SQLAlchemy or raw asyncpg for Postgres
 - Middleware chain as FastAPI dependencies: auth → rate limit → usage tracking
 - Every module must have comprehensive unit tests, including error paths and edge cases