Add GITHUB_EXCLUDED_ORGS support for organization filtering

[djmango]

Add GITHUB_EXCLUDED_ORGS Environment Variable for Organization Filtering

Problem

Users deploying gitea-mirror in environments with IP restrictions (like Proxmox/LXC) encounter sync failures when GitHub organizations have IP allowlists enabled. The entire sync process would crash with a 403 Forbidden error.

Solution

Added GITHUB_EXCLUDED_ORGS environment variable to proactively exclude specific organizations from the sync process.

Changes

• Environment Variable: GITHUB_EXCLUDED_ORGS accepts a comma-separated list of organization names to exclude
• Filtering Logic: Organizations are filtered out before any GitHub API calls are made
• Case Insensitive: Matching works regardless of casing
• Docker Support: Added to both docker-compose.yml and docker-compose.dev.yml
• Documentation: Updated docs/BUILD_GUIDE.md with usage example

Usage

# Exclude organizations with IP restrictions or that shouldn't be synced
GITHUB_EXCLUDED_ORGS=private-org,restricted-org,test-org

Benefits

• ✅ Prevents sync failures due to IP allowlists
• ✅ Gives users control over which organizations to sync
• ✅ Maintains fast parallel processing for remaining organizations
• ✅ Zero impact when not used (optional feature)

Fixes issues with IP allowlist restrictions in enterprise/restricted environments. #46