(feature) Add scheduler timezone setup to the native installer

Prompt for the scheduler timezone during interactive installs and support
a --time-zone flag for scripted installs.

Detect a sensible default from POBSYNC_TIME_ZONE, timedatectl, or
/etc/timezone before falling back to UTC, and validate the value with
Python zoneinfo before writing it to pobsync.env.

Document that schedules are evaluated in POBSYNC_TIME_ZONE so production
installs can avoid UTC/local-time ambiguity.

README.md

@@ -55,6 +55,7 @@ Common overrides:
 ```
 sudo scripts/install-systemd \
   --backup-root /mnt/backups/pobsync \
+  --time-zone Europe/Amsterdam \
   --allowed-hosts backup.example.com,localhost,127.0.0.1 \
   --csrf-trusted-origins https://backup.example.com
 ```
@@ -64,6 +65,9 @@ installer to rewrite an existing `/etc/pobsync/pobsync.env`.
 Use `--non-interactive` for scripted installs. Use `--verbose` when you want to see the underlying apt, pip, Django, and
 systemd output.
 
+Schedules are evaluated in `POBSYNC_TIME_ZONE`. The installer defaults this to the server timezone when it can detect
+one, otherwise `UTC`; override it with `--time-zone Europe/Amsterdam` or by editing `/etc/pobsync/pobsync.env`.
+
 For MariaDB support, add:
 
 ```