feat(channel): add platform-agnostic inbound access control

## Background

PR #2271 highlighted that publicly reachable channel bots can route messages from unknown users or groups into agent sessions. Telegram-specific allowlists were removed from that PR because access control should be consistent across channel implementations.

## Proposal

Add a platform-agnostic inbound access policy in the channel routing layer. The policy should be evaluated before session routing, media download, buffering, or event emission. It should support private-user and group/chat restrictions without adding platform-specific fields to individual channel configs.

## Acceptance criteria

- one shared policy model applies to all channel types
- public access is explicit and visible in configuration
- rejected events do not create sessions or download attachments
- private chats and group chats can be governed independently
- existing channel configurations have a documented compatibility path

Follow-up from #2271.

Start by reviewing PR #2271 and the channel routing layer described in this issue, then trace how channel configuration, session routing, media downloads, buffering, and event emission currently connect. Define the shared policy and compatibility path across channel types, and verify that configuration makes public access explicit while rejected private and group events stop before downstream processing.

Written by the indexing model from the issue text.