[Channels] Backend channel config and message routing

[Al629176]

Parent: #285

Add backend support to store channel config and route messages for Telegram + Discord.

Tasks

• Define/store channel config for Telegram + Discord (tokens, mappings, defaults).
• Implement JSON‑RPC / HTTP endpoints for:
  • Saving/updating channel config
  • Testing connectivity (ping/health)
• Implement outbound routing:
  • Web → Telegram
  • Web → Discord
• Implement inbound handling:
  • Telegram → OpenHuman conversation
  • Discord → OpenHuman conversation
• Ensure messages are tagged with origin (web/telegram/discord) and scoped to workspace/user.

Acceptance criteria

• Given valid config, backend can send to and receive from Telegram/Discord.
• Routing is covered by unit/integration tests.