Fix missing pool_config import issue and provide default database pool configuration.

[KarthikeyaKollu]

Description

This PR resolves the issue of a missing pool_config module in the SQLAlchemyDatabaseManager implementation (memori/database/sqlalchemy_manager.py). The absence of this module caused an ImportError.

To address the issue, I added a new pool_config.py file to centralize the default database connection pool settings. The following configurations have been introduced:

• DEFAULT_POOL_SIZE = 5: Sets the maximum size of the connection pool for regular database connections.
• DEFAULT_MAX_OVERFLOW = 10: Defines the maximum number of connections that can exceed the pool size during high-demand periods.
• DEFAULT_POOL_TIMEOUT = 30: Specifies the time (in seconds) the application waits for a database connection before throwing a timeout error.
• DEFAULT_POOL_RECYCLE = 3600: Ensures that connections are refreshed/recycled after 1 hour to prevent stale or idle connections.
• DEFAULT_POOL_PRE_PING = True: Enables pre-ping checks to validate connection health before use.

[MathiasRanna]

Noted the same issue. Would approve this pull request from my side.

Although something like this for pool_config.py would be better.

"""
Database connection pool configuration for Memori.

Provides centralized defaults for SQLAlchemy connection pool settings.
These can be overridden via environment variables or per-instance parameters.
"""

import os


class PoolConfig:
    """
    Centralized database connection pool configuration.
    
    Defaults can be overridden via environment variables:
    - MEMORI_DATABASE__POOL_SIZE
    - MEMORI_DATABASE__POOL_MAX_OVERFLOW
    - MEMORI_DATABASE__POOL_TIMEOUT
    - MEMORI_DATABASE__POOL_RECYCLE
    - MEMORI_DATABASE__POOL_PRE_PING
    """

    def __init__(self) -> None:
        """Initialize pool config with defaults or environment overrides."""
        self.DEFAULT_POOL_SIZE = int(
            os.getenv("MEMORI_DATABASE__POOL_SIZE", "5")
        )
        self.DEFAULT_MAX_OVERFLOW = int(
            os.getenv("MEMORI_DATABASE__POOL_MAX_OVERFLOW", "10")
        )
        self.DEFAULT_POOL_TIMEOUT = int(
            os.getenv("MEMORI_DATABASE__POOL_TIMEOUT", "30")
        )
        self.DEFAULT_POOL_RECYCLE = int(
            os.getenv("MEMORI_DATABASE__POOL_RECYCLE", "1800")
        )
        self.DEFAULT_POOL_PRE_PING = (
            os.getenv("MEMORI_DATABASE__POOL_PRE_PING", "true").lower() == "true"
        )


pool_config = PoolConfig()