[docs] Rework backups a bit (#1942)

* [docs] Rework backups a bit

This changes the existing backup documentation to:

* Push a bit harder on people to perform backups, it's not really just a
  nice to have
* Removes the language about migrating to/from GoToSocial and a
  different ActivityPub server since that's really not supported
* Adds a section about using backup software and provides an example on
  how to do this using Borgmatic

* [docs] Remove too much info in db section

* [docs] Add docs on how to backup local media

This adds documentation pointing people at the media list-local command
in order to determine what media files they need to include as part of
their backups.

Provides a Python script that people can use to transform the media
listing from the CLI into Borg patterns. It also includes a Borgmatic
config.yaml in the repository so people can easily fetch it instead of
copy-pasting from the docs.

* [bugfix] Ensure we emit an absolute path prefix

It works either way, as a pattern like data/files/<ID> would match a
file on /data/files/<ID>. But it would potentially also match any path
that happens to include data/files/<ID> but not rooted at the
storage-local-base-path.

* [docs] Add more links to media list CLI reference

example/borgmatic/README.md

@@ -0,0 +1,8 @@
+# Borgmatic
+
+You can find two helper files here:
+* [config.yaml](config.yaml) an example configuration for Borgmatic
+* [media-to-borg-patterns](media-to-borg-patterns.py) to generate Borg patterns for backing up local media
+
+Take a look at the [backup documentation](https://docs.gotosocial.org/en/latest/admin/backup_and_restore/) for how to use them.
+

example/borgmatic/config.yaml

@@ -0,0 +1,26 @@
+location:
+  repositories:
+    - path: ssh://<find it in your provider control panel>
+      label: <anything but typically the provider, for example borgbase>
+  patterns_from:
+  - /etc/borgmatic/gotosocial_patterns
+
+storage:
+  compression: auto,zstd
+  archive_name_format: '{hostname}-{now:%Y-%m-%d-%H%M%S}'
+  retries: 5
+  retry_wait: 30
+
+retention:
+    keep_daily: 7
+    keep_weekly: 6
+    keep_monthly: 12
+
+hooks:
+  before_backup:
+      - /usr/bin/systemctl stop gotosocial
+  after_backup:
+      - /usr/bin/systemctl start gotosocial
+  sqlite_databases:
+    - name: gotosocial
+      path: /path/to/sqlite.db

example/borgmatic/media-to-borg-patterns.py

@@ -0,0 +1,51 @@
+#!/usr/bin/env python3
+import argparse
+import os
+import pathlib
+import sys
+
+def main():
+	cli = argparse.ArgumentParser(
+		prog="media-to-borg-patterns",
+		description="""Generate Borg patterns to backup media files belonging to
+		this instance. You can pass the output to Borg or Borgmatic as a patterns file.
+		For example: gotosocial admin media list-local | media-to-borg-patterns
+		<storage-local-base-path>. You can pass a second argument, the destination file, to
+		write the patterns in. If it's ommitted the patterns will be emitted on stdout
+		instead and you can redirect the output to a file yourself.
+		""",
+		epilog="Be gay, do backups. Trans rights!"
+	)
+	cli.add_argument("storageroot", type=pathlib.Path, help="same value as storage-local-base-path in your GoToSocial configuration")
+	cli.add_argument("destination", nargs="?", type=pathlib.Path, help="file to write patterns to, or stdout if ommitted")
+	args = cli.parse_args()
+
+	output = open(args.destination, 'w') if args.destination else sys.stdout
+	# Start recursing from the storage root, including the storage root itself
+	output.write("R "+str(args.storageroot)+"\n")
+
+	prefixes=set()
+
+	for line in sys.stdin:
+		# Skip any log lines
+		if "msg=" in line:
+			continue
+		# Reduce the path to the storage path plus the account ID. By
+		# doing this we can emit path-prefix patterns, one for each account,
+		# instead of a path-file pattern for each file.
+		prefixes.add(os.path.join(*line.split("/")[:-3]))
+
+	for prefix in prefixes:
+		# Add a path-prefix, pp:, for each path we want to include.
+		output.write("+ pp:"+os.path.join(os.path.sep, prefix)+"\n")
+
+	# Exclude every file and directory under the storage root. This excludes
+	# everything that wasn't matched by any of our prior patterns. This turns
+	# the emitted patterns into an "include only" list.
+	output.write("- "+os.path.join(args.storageroot, "*")+"\n")
+
+	if output is not sys.stdout:
+		output.close()
+
+if __name__ == "__main__":
+	main()