Support Matrix server discovery (.well-known)

[wiggin77]

Summary

Implements Matrix server discovery (.well-known/matrix/server) to support deployments where the Matrix homeserver URL differs from the Matrix ID domain. Fixes #14.

Organizations commonly use Matrix server discovery to separate their homeserver location from their Matrix ID domain. For example:

• Matrix IDs: @user:example.com
• Homeserver URL: https://matrix.example.com:8008

This is configured via .well-known/matrix/server: { "m.server": "matrix.example.com:443" }

Previously, the plugin derived the Matrix ID domain directly from the homeserver URL hostname, causing Matrix IDs to be constructed incorrectly as @user:matrix.example.com instead of @user:example.com.

This is implemented with three-tier server name resolution:

1. Manual Configuration - Use explicitly configured MatrixServerName if provided
2. Automatic Discovery - Try .well-known/matrix/server discovery on the homeserver URL hostname
3. Fallback - Extract hostname from the homeserver URL (current behavior)

Ticket Link

fixes #14

[Copilot]

Pull request overview

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