chore(packaging): decide and implement what package channels do with a beta (AUR, APT / DNF, Flatpak) #1495

Part of the Beta channel epic #1499.

Problem

Today every tag reaches every package channel: AUR (pkgver cannot even contain -), the signed APT / DNF repositories, and Flatpak (.flatpak asset, and the Flathub submission). A beta must not reach people who did not ask for it, and people who did ask need a way to get it.

Decision to make (recommended option first)

1. Betas are GitHub-only at first. publish-aur and publish-linux-repo skip pre-releases (the gate is in the pre-release issue). Beta users on a package install get the zip / AppImage manually. Zero new infrastructure; package users keep Stable.
2. A separate beta line per package system: APT beta component (or suite) and DNF querya-beta repository from publish_package_repo.sh; AUR querya-desktop-beta (pkgver=0.5.0.beta1 is not allowed with -, use _ or ~-free form) with provides=querya-desktop; Flatpak beta branch on Flathub.
3. Not both at once: do 1 now, 2 when there is demand.

Scope (for option 2, kept here so the choice is informed)

• scripts/linux/publish_package_repo.sh: parameter for the component / repo name; sign and publish under beta, and never touch the stable index from a beta tag.
• packaging/linux/aur/PKGBUILD template for querya-desktop-beta, conflicts=('querya-desktop'), source URL with the tag form.
• Flatpak: --branch=beta in build_flatpak.sh and in the Flathub manifest once feat(packaging): publish Querya Desktop to Flathub repository #1060 is accepted.
• docs/packaging.md: how to subscribe to the beta line and how to leave it.

Acceptance

• Documented decision in docs/packaging.md.
• For the chosen option: a beta tag leaves the stable repositories and the AUR querya-desktop package unchanged (verified by a dry run).