The user wants a complete single-file FastAPI application with 5 resources (books, authors, publishers, genres, customers), each with CRUD endpoints (list, get by id, create, update, delete). Let me plan this out:

Resources:
1. books - fields: title (str), isbn (str), price (float), publication_year (int)
2. authors - fields: first_name (str), last_name (str), birth_year (int), nationality (str)
3. publishers - fields: name (str), country (str), founded_year (int), website (str)
4. genres - fields: name (str), description (str), parent_genre (str|None), created_at (str)
5. customers - fields: name (str), email (str), phone (str), joined_date (str)

Each needs:
- Pydantic model (create + possibly read)
- In-memory dict storage
- Integer id from a counter
- 5 endpoints: GET list, GET by id, POST create, PUT update, DELETE

I'll use a pattern with counters per resource. Let me write clean, type-hinted code with one-line docstrings.

Let me structure this:
- Imports
- Pydantic models (5)
- Storage dicts and counters
- Endpoints (25 total)

I need to be careful: "No comments about what you might do differently" - so no speculative comments. Just the code.

Let me write it out.