feat: introduce ODMClient for simplified multi-database management

[A-K-6]

Problem Statement

Managing multiple MongoDB databases and their associated Beanie models often requires significant boilerplate code to handle connections, migrations, and initialization for each database individually.

Solution: ODMClient

This PR introduces the ODMClient, a high-level utility class that centralizes and simplifies the lifecycle of Beanie applications.

Key Features

• Batch Initialization: Use init_db with a configuration dictionary to initialize multiple databases and models in one call.
• Centralized Connection Management: Uses AsyncMongoClient internally and supports async context manager usage (async with ODMClient(...) as client:).
• Integrated Migrations: Automatically handles migrations during database registration with a built-in lock for safety.
• Full Feature Support: Passes through all standard Beanie init parameters (allow_index_dropping, recreate_views, skip_indexes).
• Improved Documentation: Includes a new tutorial and automated API documentation generation.

Examples

from beanie import ODMClient, Document

class User(Document): ...
class Product(Document): ...

db_config = {
    "user_db": [User],
    "product_db": [Product]
}

async with ODMClient("mongodb://localhost:27017") as client:
    await client.init_db(db_config, migrations_path="path/to/migrations")

Checklist

• Code implementation of ODMClient (beanie/odm/client.py)
• Comprehensive tests in tests/odm/test_client.py
• Documentation added in docs/odm-client.md
• mkdocs.yml / pydoc-markdown.yml updated for navigation
• Tests for ODMClient passing locally

[roman-right]

Interesting idea, but this is a large feature addition that needs more discussion before merging. A few concerns:

1. The ODMClient introduces a new high-level abstraction - we need to be sure this is the right API surface before committing to it.
2. Migration handling built into the client adds complexity. Users who do not need migrations still get that code path.
3. The PR branches from main to main which makes the diff harder to review.

Could you open a discussion issue first so we can align on the API design? I would rather get the interface right before reviewing the implementation details.

[A-K-6]

Thank you for your patience. Since I opened this PR, there have been two major internet shutdowns in my country - Iran - , and we are currently in the middle of the second one. My internet access is extremely limited and unstable at the moment. I will address the remaining items as soon as I have a stable connection again. Thanks for understanding.