refactor: refactor monolithic api.py and services.py into modular structure

[Copilot]

Split monolithic api.py (349 lines) and services.py (248 lines) into focused modules organized by domain responsibility.

Structure

API modules (fastpubsub/api/):

• app.py - create_app() factory function
• server.py - Gunicorn server configuration
• middlewares.py - HTTP middleware
• helpers.py - Error response utilities
• routers/ - Route handlers by domain (topics, subscriptions, monitoring)

Service modules (fastpubsub/services/):

• topics.py - Topic CRUD operations
• subscriptions.py - Subscription CRUD operations
• messages.py - Message publish/consume, DLQ, metrics
• helpers.py - Shared database helpers

Test modules mirror production structure:

• tests/api/routers/ - API endpoint tests by router
• tests/services/ - Service layer tests by domain
• tests/helpers.py - Shared test utilities

Usage

# Backward compatible - existing imports work unchanged
from fastpubsub.api import app, run_server
from fastpubsub import services

# New modular imports available
from fastpubsub.api.app import create_app
from fastpubsub.services.topics import create_topic

All 83 tests pass with maintained 91% coverage.

Original prompt

I want you to create some modules to separate the responsibilities we currently have in the project.

What I expect to find related to api.py:

api/init.py
api/app.py -> here, there should be a create_app function that will return the app fully configured.
api/server.py -> here, there should be things related to the server (CustomGunicornApp and run_server).
api/middlewares.py -> middlewares used.
api/helpers.py -> any helper that is used.
api/routers/init.py
api/routers/topics.py -> routes with the topics tag
api/routers/subscriptions.py -> routes with the subscriptions tag
api/routers/monitoring.py -> routes with the monitoring tag

What I expect to find related to services.py:

services/init.py
services/topics.py -> everything related to topics.
services/subscriptions.py -> everything related to subscriptions.
services/messages.py -> everything related to message publishing and consumption.
services/helpers.py -> any helper that is used.

I also want the tests to be refactored to follow the same hierarchy as the new modules.

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