feat(packaging): COPR repository for Fedora

The spec builds from the tag archive and a vendored crates tarball when
git_dir is not defined, so COPR can build it without network access.
release.yml publishes the vendor tarball and submits the build to COPR
after the release is published.

Refs #42

packaging/copr/README.md

@@ -0,0 +1,43 @@
+# COPR repository
+
+Fedora users install from the COPR project
+[`narrrl/proton-drive-linux`](https://copr.fedorainfracloud.org/coprs/narrrl/proton-drive-linux/),
+so `dnf upgrade` brings every release. COPR builds `packaging/proton-drive-linux.spec` from two
+release assets:
+
+| Source | What it is |
+| --- | --- |
+| `Source0` | GitHub's archive of the `v<version>` tag |
+| `Source1` | `proton-drive-linux-vendor-<version>.tar.xz`: every crate from `Cargo.lock`, made by `cargo vendor` in `release.yml` |
+
+With the vendored crates the build needs no network, so the COPR project keeps network access
+off.
+
+## Releasing a new version
+
+`.github/workflows/copr.yml` does this automatically:
+
+- **On a release tag**, `release.yml` calls it after the GitHub release is published. It writes
+  the version into the spec, downloads both sources with `spectool`, builds a source package
+  and submits it with `copr-cli build`. The job waits for COPR, so a failed build fails the
+  release workflow.
+- **By hand**, run the workflow from the Actions tab with a version. The release must already
+  have its vendor tarball.
+
+It needs the `COPR_CONFIG` repository secret: the whole content of `~/.config/copr` from
+<https://copr.fedorainfracloud.org/api/>. **The token expires after 180 days.** Renew it on that
+page and replace the secret, or the next release's COPR job fails.
+
+To do the same by hand:
+
+```bash
+spectool -g -C ~/rpmbuild/SOURCES packaging/proton-drive-linux.spec
+rpmbuild -bs packaging/proton-drive-linux.spec
+copr-cli build narrrl/proton-drive-linux ~/rpmbuild/SRPMS/proton-drive-linux-*.src.rpm
+```
+
+## Project settings
+
+- Chroots: `fedora-44-x86_64`, `fedora-45-x86_64` and `fedora-rawhide-x86_64`. The spec is
+  `ExclusiveArch: x86_64` and needs `rust >= 1.96`.
+- Network access in builds: off.