feat(channels): add channel schema, RPC controllers, and definition-driven UI

[senamakel]

Summary

• Add a proper channel definition schema at the Rust core level with types for auth modes (API key, bot token, OAuth, managed DM), field requirements, and capabilities
• Implement 6 RPC controllers in the channels namespace: list, describe, connect, disconnect, status, test
• Wire the UI (MessagingPanel) to be fully backend-driven — channel list, auth modes, and input fields all come from the Rust definitions
• Support 3 channels: Telegram, Discord, Web
• Include 21 unit tests covering definitions, validation, and schema/handler parity
• Add fallback definitions so the UI works even when the core sidecar is unreachable

Key design decisions

• auth_action field: OAuth/managed auth modes return a descriptor (e.g. "discord_oauth", "telegram_managed_dm") so the frontend knows which auth flow to initiate
• Credential storage: Reuses the existing credentials domain with provider key channel:{id}:{mode}
• restart_required: true: Credential-based connections require a service restart to activate the channel listener (hot-reload can come later)
• Fallback definitions: If the backend is unreachable, the UI renders hardcoded definitions so users can still see and interact with channels

New files

File	Purpose
src/openhuman/channels/controllers/mod.rs	Module re-exports
src/openhuman/channels/controllers/definitions.rs	Channel types + static registry (Telegram, Discord, Web)
src/openhuman/channels/controllers/ops.rs	Business logic for channel lifecycle
src/openhuman/channels/controllers/schemas.rs	RPC controller wiring (6 methods)

Test plan

• cargo check — zero warnings
• cargo test --lib channels::controllers — 21 tests pass
• yarn typecheck — clean
• yarn lint — clean
• yarn format:check — clean
• Manual: open Settings → Messaging, verify channels render with correct auth modes and fields
• Manual: enter a bot token, click Connect, verify credentials stored and restart message shown

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