feat: add support for custom OpenAI configurations

[rajubeparybd]

What kind of change does this PR introduce?

Feature: Add environment variable configuration for OpenAI settings to enable custom API endpoints and models

Why was this change needed?

This change allows users to configure custom OpenAI-compatible endpoints and models through environment variables, enabling integration with alternative AI providers like Ollama, OpenRouter, or other OpenAI-compatible APIs. Previously, the application was hardcoded to use OpenAI's specific models and endpoints, limiting flexibility for users who want to use different AI providers.

The changes include:

• OPENAI_BASE_URL: Allows custom base URL for OpenAI-compatible APIs
• OPENAI_CHAT_MODEL: Configurable chat model (defaults to gpt-4.1)
• OPENAI_IMAGE_MODEL: Configurable image generation model (defaults to dall-e-3)

This maintains backward compatibility while providing the flexibility needed for different deployment scenarios.

#1074

Other information:

This change affects multiple services across the backend including:

• Agent graph services
• Autopost service
• Copilot controller
• OpenAI service
• Chat tools service

No breaking changes - all existing functionality is preserved with sensible defaults.

Checklist:

Put a "X" in the boxes below to indicate you have followed the checklist.

• I have read the CONTRIBUTING guide.
• I checked that there were not similar issues or PRs already open for this.
• This PR fixes just ONE issue (do not include multiple issues or types of change in the same PR) For example, don't try and fix a UI issue and include new dependencies in the same PR.

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