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.

pyproject.toml

@@ -8,9 +8,15 @@ version = "0.1.0"
 description = "Pull-based rsync backup tool with hardlinked snapshots"
 requires-python = ">=3.11"
 dependencies = [
+  "Django>=5.2,<6.0",
   "PyYAML>=6.0"
 ]
 
+[project.optional-dependencies]
+mariadb = [
+  "mysqlclient>=2.2"
+]
+
 [project.scripts]
 pobsync = "pobsync.cli:main"