fix(channels): Nextcloud Talk never delivers messages to the agent

[guzzi235]

Summary

Two independent bugs combine to make the Nextcloud Talk channel receive messages but never forward them to the agent:

1. Long-poll timeout race. The polling HTTP client used a 30s timeout, but Nextcloud's lookIntoFuture=1 long-poll can legitimately take up to ~30.7s to return when there are no new messages (measured directly against a real instance: HTTP_CODE:304 TIME:30.694274s). The client aborted just before the server replied, surfacing as a permanent error sending request on every poll cycle, for every room. Raised the timeout to 45s for headroom.

2. Missing @mention support trips the group-chat filter. Every incoming message is constructed with is_group: true. The bridge's routing (bridge.rs) silently drops group messages that don't @mention the bot ("Ignoring group message ... not mentioned"), but the Nextcloud adapter never implements mention detection — so every message was dropped regardless of the timeout fix. Nextcloud Talk conversations used with this adapter are 1:1 bot<->operator rooms, so is_group: false matches actual usage and lets messages through.

Test plan

• Verified end-to-end against a real Nextcloud Talk instance (self-hosted).
• Without these two fixes: the bot authenticates and polls successfully but never responds to any message.
• With both fixes applied: the bot responds reliably to messages sent in the room, confirmed over multiple send/reply cycles including after container restarts.

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