v0.82.0: Backup extensions

Extensions Uchiyomi added by itself are backups now. They help while a series needs them, then go, and come back
when the series is stuck again. Discover shows only the sources you added, and the extension engine doesn't keep
extensions nothing uses.

Backup extensions

• Not in Discover. An extension Uchiyomi installed by itself (Fix everything does, when no installed source has a
  series) isn't shown in Discover or anywhere you pick a source. Your series still name it where a chapter came from.
• Removed when unused. It stays while a series needs it, and is removed after 3 days without use. It is never
  removed while a series uses it as its main source.
• Back when needed. When a series it carried is stuck again for an hour or more, it is put back and the series
  carries on, without searching every extension again.
• New chapters come from the main source while that source works; a backup's copy is taken only when the main is
  slowed or failed that chapter.
• At most 5 at once. The one needed longest ago makes room; one in use never does.
• Yours if you want it. Admin → Sources → Added by Uchiyomi lists them; Keep makes one yours. Installing
  one yourself, switching its sources on or choosing it for a series does the same. One you remove is never installed
  again, by Fix everything or anything else.
• One install at a time. An extension you add or remove while another install runs waits for it, and says so if
  it takes more than two minutes.

Upgrading

• Your extensions: the ones Fix everything installed, and that nobody chose for a series since, become backups.
  Each gets three days before anything is removed. An extension you removed yourself is remembered.
• One optional knob, BACKUP_EXTENSIONS_MAX (default 5, 0 turns backups off), is in
  docs/CONFIGURATION.md under advanced, rarely needed.
• Database: two new tables (backup_extensions, backup_links) and one new column on series_scout. All additive.

Uchiyomi Desktop (beta): downloads

Uchiyomi as a program on your own computer, for Windows and Mac. On first launch it asks how you want to
use it: on this computer (the whole app, with the library in a folder on your PC and no Docker or
server to run) or connected to your own Uchiyomi server (a window onto the server you already run).

Your computer	Download
Windows (x64)	Uchiyomi-Setup-0.82.0.exe
Mac with Apple silicon (M1 or newer)	Uchiyomi-0.82.0-arm64.dmg
Mac with an Intel processor	Uchiyomi-0.82.0-x64.dmg

Only those three are for you: the .zip, .blockmap and latest*.yml files are what the app's own updater
reads. The same installers are also here under names without a version (Uchiyomi-Setup.exe,
Uchiyomi-mac-arm64.dmg, Uchiyomi-mac-x64.dmg), which always lead to the newest release.

The app is not signed yet, so the first launch takes one extra step:

• Windows: "Windows protected your PC" appears once. Choose More info, then Run anyway. A PC
  with Smart App Control turned on blocks unsigned apps with no way past it; use the Docker install there.
• macOS: open the dmg and drag Uchiyomi to Applications. The first time you open it, macOS refuses;
  go to System Settings → Privacy & Security and choose Open Anyway.

Windows installs updates by itself. A Mac cannot update an unsigned app, so there a new version is a new
download; the menu-bar icon and Admin → Health say when one is out.

Step by step, with pictures -- which file, the first launch on each system, the two ways to use it, adding
sources, updates, backups, where files live and uninstalling:
the desktop guide.