Prepare the 0.1.0-alpha.1 release

info.xml's version field is restricted by Nextcloud's own info.xsd to plain
numeric SemVer, no pre-release suffix, so the alpha/beta/rc stage from
PLAN.md's release table lives in the git tag and CHANGELOG instead; noted
that in PLAN.md so it isn't rediscovered later. Bumped the version, wrote
an honest description for an alpha (feature-complete for phase 1, not for
production), and added CHANGELOG.md.

README: new "Storage backend experiences" section, seeded with the Hetzner
Storage Box sub-account root-path quirk found while testing a real SFTP
target (root must be "home", not "/"), open for others to extend via PR.

CHANGELOG.md

@@ -0,0 +1,44 @@
+# Changelog
+
+All notable changes to NG Backup are documented here. The format follows
+[Keep a Changelog](https://keepachangelog.com/en/1.1.0/); versioning follows the stages in
+`PLAN.md` section 9. The `<version>` in `appinfo/info.xml` is plain SemVer-numeric (Nextcloud's
+own `info.xsd` doesn't allow a `-alpha`/`-beta`/`-rc` suffix there); the stability stage is instead
+carried by the git tag, the release, and this file.
+
+## [0.1.0-alpha.1] - 2026-10-05
+
+First public alpha. Feature-complete for phase 1: scheduled, encrypted, incremental backups, and
+restore of files and folders. **Not for production use** — test on a copy of production (dev,
+test, staging). Full disaster recovery (`backup:restore:full` on a fresh installation) is phase 2
+and not implemented yet.
+
+### Added
+- Encrypted (XChaCha20-Poly1305), content-addressed, deduplicated, incremental backups, streamed
+  from source to destination without a local copy of the data.
+- Backup locations via files_external's own backend/auth classes, without ever mounting the
+  location: S3 (and compatible services), SFTP, WebDAV, FTP, Swift, or a local path.
+- A recovery kit with up to three passphrase slots, mandatory download-and-confirm before the
+  first backup runs, and an optional encrypted copy by email.
+- Resumable, time-boxed background jobs (database dump, file walk, upload), so a run survives
+  short PHP time limits and a crash is resumed, never restarted, without leaving the instance in
+  maintenance mode.
+- Database dump and restore without external tools, via Nextcloud's own `IDBConnection`/DBAL,
+  streamed through the same pipeline as file data.
+- Restore of files and folders via the web interface and `occ`, into a new folder, merged, or
+  replacing the original (with the previous content going to the trash).
+- Retention policy (last/daily/weekly/monthly), prune with pack repacking, and a snapshot trash
+  with a configurable deletion delay.
+- Append-only locations and documented recommendations against ransomware and a compromised
+  server (object lock, delete-less credentials, off-site copies).
+- Admin settings page (locations, schedule, retention, key management) and a JSON API.
+- Notifications and an admin setup check for a failed or overdue backup.
+- CI: unit tests on PHP 8.2–8.4, and an end-to-end backup→restore round trip on Nextcloud 32–35 ×
+  SQLite/MariaDB/PostgreSQL, and against real S3 and SFTP backends (not just a local path).
+
+### Known limitations
+- No full/disaster-recovery restore yet (phase 2).
+- No per-user backup/restore via user_migration yet (phase 2).
+- No FTP/SMB coverage in CI yet, and no verification/checksum-audit command yet (phase 2).
+- See the "Storage backend experiences" section of the README for provider-specific quirks found
+  so far (e.g. Hetzner Storage Box sub-accounts).