(release) Prepare 1.0.0 release metadata

Add the initial 1.0.0 changelog, bump the package/application version,
and expose the release version through `pobsync --version`.

Cover the version output in the console entrypoint tests.

src/pobsync_backend/tests/test_console_entrypoint.py

@@ -9,6 +9,14 @@ from pobsync.cli import main
 
 
 class ConsoleEntrypointTests(SimpleTestCase):
+    def test_version_prints_package_version(self) -> None:
+        stdout = StringIO()
+        with patch("sys.stdout", stdout):
+            exit_code = main(["--version"])
+
+        self.assertEqual(exit_code, 0)
+        self.assertEqual(stdout.getvalue().strip(), "pobsync 1.0.0")
+
     def test_maps_backup_alias_to_django_command(self) -> None:
         with patch("pobsync.cli.execute_from_command_line") as execute:
             exit_code = main(["backup", "web-01", "--dry-run"])