feat: add Django backend foundation and Docker runtime

Add a Django admin-backed management layer for pobsync configs, runs,
snapshots, and schedules. Keep the existing CLI engine as the execution
source of truth, add import/run management commands, and provide SQLite
default plus optional MariaDB Docker Compose support.

README.md

@@ -100,9 +100,11 @@ Cron output is redirected to:
 
 ## Development (optional)
 
-For development purposes you can still use an editable install, this is why pyproject.toml still exists:
+For development purposes you can still use an editable install, this is why pyproject.toml still exists. On systems with an externally managed Python installation, create a virtualenv first.
 
 ```
+python3 -m venv .venv
+. .venv/bin/activate
 python3 -m pip install -e .
 pobsync --help
 ```
@@ -110,3 +112,64 @@ pobsync --help
 For production use, always use the canonical entrypoint:
 
 /opt/pobsync/bin/pobsync
+
+## Django backend (early refactor layer)
+
+The Django backend is a management layer around the existing pobsync engine. The current CLI remains the source of truth for executing backups; Django stores configs, schedules, backup runs, and snapshot metadata so the project can grow toward a web/admin/API surface without rewriting rsync behavior in one risky step.
+
+### Local SQLite development
+
+```
+python3 -m venv .venv
+. .venv/bin/activate
+python3 -m pip install -e .
+mkdir -p var
+python3 manage.py migrate
+python3 manage.py createsuperuser
+python3 manage.py runserver
+```
+
+The admin is available at:
+
+- http://127.0.0.1:8000/admin/
+
+Import existing YAML configs into the database:
+
+```
+python3 manage.py import_pobsync_configs --prefix /opt/pobsync
+```
+
+Run a backup through Django while still using the existing pobsync engine:
+
+```
+python3 manage.py run_pobsync_backup <host> --prefix /opt/pobsync --prune
+```
+
+### Docker with SQLite
+
+```
+docker compose up --build web
+```
+
+This starts Django on:
+
+- http://127.0.0.1:8000/admin/
+
+The container persists `/opt/pobsync` and the SQLite database in Docker volumes.
+
+### Docker with MariaDB
+
+```
+docker compose --profile mariadb up --build web-mariadb
+```
+
+The MariaDB profile is optional. SQLite remains the default because it is enough for a single backup server and keeps deployment simple.
+
+### Refactor direction
+
+Recommended next steps:
+
+- Move config reading/writing behind a repository interface that can use YAML or Django models.
+- Record `run-scheduled` results into `BackupRun`.
+- Add a snapshot discovery command that syncs existing snapshot metadata into `SnapshotRecord`.
+- Add tests around retention, scheduling, and config merge before deeper internal reshaping.