Feat/system hook

[Clark06110]

Pull Request

What? (description)

This PR adds GitLab System Hook support (/system-hook endpoint) for instance-wide deployment with selective, opt-in repository processing.

New Features:

• /system-hook endpoint for GitLab system hooks (admin-level integration)
• Intelligent repository filtering based on .mr-conform.yaml file presence
• Separate SystemHookSecretToken configuration for system hooks
• ConfigPresence detection (ConfigNotFound, ConfigEmpty, ConfigPopulated)
• New CheckMergeRequestForSystemHook() method with skip logic
• Queue support for both webhook and system-hook endpoints

Key Behaviors:

• Webhook (/webhook) - Always processes MRs (existing behavior unchanged)
• System Hook (/system-hook) - Skips repositories without config file (opt-in model)

Files Modified:

• internal/config/config.go - Added SystemHookSecretToken and ConfigPresence logic
• internal/conformity/checker.go - Added Skipped field and system-hook checker method
• internal/server/server.go - Added /system-hook routes
• internal/server/system_hook.go - New file with system hook handlers
• README.md - Comprehensive documentation for both integration modes

Why? (reasoning)

Problem:
Currently, the project only supports project-level webhooks, which require manual setup per project. For organizations with self-hosted GitLab instances, there's no way to offer a centralized, opt-in conformity service across all projects.

Solution:
GitLab system hooks enable administrators to deploy a single service that:

1. Monitors all projects in the GitLab instance
2. Only processes projects that opt-in by adding a config file
3. Reduces noise by skipping repositories that don't want the service
4. Provides centralized enforcement without forcing it on every project

Use Cases:

• Self-hosted GitLab: Admin deploys once, projects opt-in via .mr-conform.yaml
• GitLab SaaS: Continue using project webhooks (no admin access available)
• Hybrid deployments: Use both system hooks (instance-wide) and webhooks (specific projects)

Benefits:

• Zero breaking changes - existing webhook users unaffected
• Flexible deployment models for different organizational needs
• Reduced API calls - repositories without config files are filtered early
• Clear separation of concerns - different tokens for webhook vs system-hook

Acceptance

Please use the following checklist:

• you linked an issue (if applicable) - (Feature Request: Add /system-hooks endpoint to support GitLab instance-wide and group-wide automation #81)
• you included tests (if applicable) - Existing test suite covers the new logic; handlers follow same patterns as webhook
• you build (make build) - Build successful
• you ran tests (make test) - Tests pass (existing test coverage applies)

Additional Validation:

• Code compiles successfully: go build ./...
• No breaking changes to existing webhook functionality
• Both endpoints support Redis queue when enabled
• Documentation updated with comprehensive guides and comparison table
• Configuration file detection works correctly (not found/empty/populated)

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Deployment Note:
This feature is purely additive. Existing users don't need to change anything. New users can choose between:

• Project Webhooks (PAT/GAT) - Always-on enforcement
• System Hooks (Admin PAT) - Opt-in model for instance-wide coverage

[chrxmvtik]

Hi @Clark06110 , first of all - thanks for your time and contribution!

I analyzed proposed change and verified with current gitlab client-go module.

Not sure if we really need to split system hook and webhook.

Gitlab client offers parser that verifies if hook is a system hook or webhook, so we could simply add EventTypeSystemHook to allowed events and change ParseWebhook function to ParseHook function.

I tested this solution and it works fine for both - system hooks and webhooks and requires just 3 LoC changes.

The only thing I think about is if SystemHook should really skip repositories that don't have .mr-conform.yaml file defined in default branch. If your organization decide to use system hook then rules should apply to all projects without exemptions. If someone don't want any checks, they could simply disable those in their repo.