fix(backend): Add the lower cap of the user credits to zero

[majdyz]

Changes 🏗️

Add the lower cap of the user credits to zero.

Checklist 📋

For code changes:

• I have clearly listed my changes in the PR description
• I have made a test plan
• I have tested my changes according to the test plan:
  • ...

Example test plan

• Create from scratch and execute an agent with at least 3 blocks
• Import an agent from file upload, and confirm it executes correctly
• Upload agent to marketplace
• Import an agent from marketplace and confirm it executes correctly
• Edit an agent from monitor, and confirm it executes correctly

For configuration changes:

• .env.example is updated or already compatible with my changes
• docker-compose.yml is updated or already compatible with my changes
• I have included a list of my configuration changes in the PR description (under Changes)

Examples of configuration changes

• Changing ports
• Adding new services that need to communicate with each other
• Secrets or environment variable changes
• New or infrastructure changes such as databases

[qodo-merge-pro]

PR Reviewer Guide 🔍

Here are some key observations to aid the review process:

⏱️ Estimated effort to review: 1 🔵⚪⚪⚪⚪

🧪 No relevant tests

🔒 No security concerns identified

⚡ Recommended focus areas for review Logic Change The change prevents negative credit values from being shown to users by using max(credit,0). Verify this doesn't hide important information about actual credit state from users or admins.

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