Add a Discord notification channel adapter

[jstuart0]

Goal

Deliver HITL requests to a Discord channel via webhook + interactive buttons, mirroring the Slack/Telegram flows.

Background

Channel adapter interface: src/server/services/channels/types.ts. Reference impl: telegram.ts. Discord's webhook API is the simplest send path; for interactive buttons you need a bot user and interaction endpoints with ed25519 signature verification.

Scope

• src/server/services/channels/discord.ts — send via webhook, format proposal as an embed with two button components.
• src/server/routes/channels.ts — POST /api/v1/channels/discord/interactions with ed25519 verification (Discord sends X-Signature-Ed25519 + X-Signature-Timestamp). Route the button press to resolveHitlRequest.
• Credential storage: encrypted JSON with {webhookUrl, publicKey, botToken?} in notification_channels.credential_ciphertext.
• UI: SlackChannelPanel.tsx equivalent for Discord (paste webhook, paste public key, enroll + test).

Acceptance criteria

• Settings wizard accepts a Discord webhook URL and application public key.
• HITL proposal delivered to Discord renders as an embed with Approve / Decline buttons that resolve the proposal.
• Ed25519 signature verification rejects replay + forged requests.
• Test coverage for signature verification + interaction payload handling.