[config] validate gateway mode

[capcom6]

Confidence Score: 5/5

The PR appears safe to merge; invalid and explicitly empty gateway modes now fail startup instead of producing inconsistent behavior.

Summary

Validates gateway.mode while constructing push configuration so only public and private are accepted and invalid values fail startup.

• Maps public mode to FCM and private mode to the upstream push provider.
• Adds a reusable invalid-configuration sentinel error.
• Documents the gateway-mode validation and updates example environment settings.
• The prior empty-mode finding is resolved because the default switch branch now rejects empty values.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Load gateway mode] --> B{Mode}
    B -->|public| C[Use FCM push]
    B -->|private| D[Use upstream push]
    B -->|empty or unsupported| E[Return invalid config error]
    E --> F[Fail application startup]

Loading

_{Reviews (4) · Last reviewed commit: "[docs] cleanup readme"}

[github-actions]

🤖 Pull request artifacts

Platform	File
🐳 Docker	GitHub Container Registry
🍎 Darwin arm64	server_Darwin_arm64.tar.gz
🍎 Darwin x86_64	server_Darwin_x86_64.tar.gz
🐧 Linux arm64	server_Linux_arm64.tar.gz
🐧 Linux i386	server_Linux_i386.tar.gz
🐧 Linux x86_64	server_Linux_x86_64.tar.gz
🪟 Windows arm64	server_Windows_arm64.zip
🪟 Windows i386	server_Windows_i386.zip
🪟 Windows x86_64	server_Windows_x86_64.zip