Clean up native OTR runtime dependencies

[Ri0n]

Replacement merge PR for draft #147. This branch points at the exact same green head SHA 60207a1ae9f8f9f87e782b17d96a1ad2a586528c.

Completes the post-switch cleanup of the native qca-otr backend:

• remove libtidy and obsolete legacy dependency find modules
• remove dead libotr-era runtime glue
• keep qca-otr as the native runtime backend
• preserve meaningful GPL provenance/comments in existing plugin code
• license independently implemented qca-otr under MIT with Sergei Ilinykh copyright
• add Doxygen to public qca-otr APIs
• retain Qt5/Qt6 Linux and Qt5/Windows 7 CI coverage