docs: add backup/restore guide for odysseus-backup (#2587)

The scripts/odysseus-backup snapshot/restore CLI was undocumented in
README.md and docs/. Add docs/backup-restore.md covering the snapshot,
list, verify, and restore subcommands, default include/skip behavior
(deep_research and mail-attachments skipped unless flagged), the
destructive-restore warning and its data.before-restore-* stash, a cron
example, and Docker-vs-native data/ paths (including the ChromaDB named
volume caveat). Link it from the README Data section.

Addresses the "Backup/restore guide and helper flow for data/" item in
ROADMAP.md. Docs only; no change to the tool.

Fixes #2583

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/backup-restore.md

@@ -0,0 +1,129 @@
+# Backup & Restore
+
+Odysseus keeps all of your state in the `data/` directory — the SQLite database
+(`app.db`), the Fernet encryption key (`data/.app_key`), the vault, memory, RAG
+indexes, personal documents, and uploads. The `scripts/odysseus-backup` tool
+snapshots that directory into a single gzip tarball and restores it later.
+
+Snapshots are safe to take while the app is running: SQLite databases are copied
+through SQLite's own `.backup` API rather than a raw file copy, so an in-flight
+write can't corrupt the snapshot.
+
+> **A snapshot contains your secrets.** The tarball includes the Fernet
+> encryption key (`data/.app_key`), the vault, sessions, and any stored
+> provider/API tokens — so treat it like a password. Store backups somewhere
+> private, never commit them to Git, and prefer an encrypted destination when
+> copying them offsite.
+
+## Quick start
+
+Run the tool from the repository root:
+
+```bash
+# Create a snapshot → backups/odysseus-backup-<YYYYMMDD-HHMMSS>.tar.gz
+./scripts/odysseus-backup snapshot
+
+# List existing snapshots (most recent first)
+./scripts/odysseus-backup list
+
+# Check a tarball's integrity without extracting it
+./scripts/odysseus-backup verify backups/odysseus-backup-20260101-120000.tar.gz
+
+# Restore (destructive — see the warning below)
+./scripts/odysseus-backup restore backups/odysseus-backup-20260101-120000.tar.gz --yes
+```
+
+The script depends only on the Python standard library, so any `python3` on your
+`PATH` will run it — you don't need the app's virtualenv active.
+
+Every command prints a JSON result. Add `--pretty` for indented output.
+
+## Commands
+
+### `snapshot`
+
+Writes a `tar.gz` of `data/` to `backups/<timestamp>.tar.gz`.
+
+| Flag | Effect |
+| --- | --- |
+| `--out PATH` | Write to a specific path instead of the default `backups/` location. Must be **outside** `data/`. |
+| `--include-research` | Include `data/deep_research/` (skipped by default — research runs are large). |
+| `--include-attachments` | Include `data/mail-attachments/` (skipped by default — cached IMAP extractions, re-derivable). |
+
+By default the snapshot includes everything under `data/` **except**
+`deep_research/` and `mail-attachments/`. Personal uploads and documents are
+included.
+
+```bash
+# Snapshot straight to a mounted NAS path
+./scripts/odysseus-backup snapshot --out /mnt/nas/odysseus-$(date +%F).tar.gz
+
+# Full snapshot including research runs and mail attachments
+./scripts/odysseus-backup snapshot --include-research --include-attachments
+```
+
+### `list`
+
+Lists the tarballs in `backups/`, most recent first, with size and modification
+time.
+
+### `verify PATH`
+
+Opens the tarball read-only and walks every member to confirm it is intact and
+safe to restore. Nothing is extracted. Use this before relying on an old backup
+or after copying one across machines.
+
+### `restore PATH --yes`
+
+Overwrites `data/` from a tarball.
+
+> **Restore is destructive.** It replaces the current `data/` directory. `--yes`
+> is required so a mistyped command can't wipe your live state.
+
+Restore is not a blind delete: before extracting, the tool **renames your current
+`data/` to `data.before-restore-<timestamp>`** in the repository root. If a
+restore turns out to be wrong, your previous state is still there — delete the
+restored `data/` and rename the stashed directory back. The restore path is also
+validated entry-by-entry: archives containing absolute paths, `..` segments,
+symlinks, or anything outside `data/` are rejected.
+
+## Scheduling offsite backups
+
+The tarball output composes cleanly with cron and any copy tool. For example, a
+nightly snapshot copied offsite:
+
+```cron
+0 3 * * *  cd /path/to/odysseus && ./scripts/odysseus-backup snapshot --out "/mnt/nas/odysseus-$(date +\%F).tar.gz"
+```
+
+Swap the `--out` target for `scp`, `rclone`, `s3cmd`, or similar to push the
+snapshot to remote storage.
+
+## Docker vs native installs
+
+The tool reads `data/` and writes `backups/` relative to the repository root, so
+where you run it matters:
+
+- **Native installs** — run it from the repo root as shown above. `data/` and
+  `backups/` are both in the repo directory.
+- **Docker** — `docker-compose.yml` bind-mounts the host's `./data` to
+  `/app/data`, so the live data is also present on the host. **Run the tool on
+  the host** from the repo root; the snapshot reads the bind-mounted `./data` and
+  writes to `./backups` on the host. Running it *inside* the container is not
+  recommended, because `backups/` is not a mounted volume and the tarball would
+  be lost when the container is recreated.
+
+> **ChromaDB caveat (Docker only).** In the Docker setup, ChromaDB stores its
+> vectors in a separate Compose-managed volume (declared as `chromadb-data`),
+> **not** under `./data`. `odysseus-backup` therefore does not capture the Docker
+> ChromaDB store. Back it up separately if you need it. Compose prefixes the
+> volume with the project name, so find the real name first
+> (`docker volume ls | grep chromadb`), then archive it — for example:
+>
+> ```bash
+> docker run --rm -v <project>_chromadb-data:/data -v "$PWD":/backup \
+>   alpine tar czf /backup/chromadb.tar.gz -C /data .
+> ```
+>
+> On native installs ChromaDB lives at `data/chroma/` and is included in the
+> snapshot normally.