(feature) Capture structured backup statistics

Parse rsync --stats output into structured run metrics for file counts,
transferred bytes, literal data, matched data, speedup, and estimated
link-dest savings.

Store collected stats on backup run results and successful snapshot metadata,
including snapshot data usage and backup-root capacity details for future
dashboard graphs and disk-full projections.

Render the collected metrics on run and snapshot detail pages, with tests
covering parsing, metadata persistence, and UI output.

src/pobsync_backend/tests/test_run_stats.py

@@ -0,0 +1,60 @@
+from __future__ import annotations
+
+import os
+from pathlib import Path
+from tempfile import TemporaryDirectory
+
+from django.test import SimpleTestCase
+
+from pobsync.run_stats import parse_rsync_stats, tree_usage
+
+
+class RunStatsTests(SimpleTestCase):
+    def test_parse_rsync_stats_extracts_counts_bytes_and_savings(self) -> None:
+        stats = parse_rsync_stats(
+            """
+Number of files: 1,234 (reg: 1,200, dir: 34)
+Number of created files: 12 (reg: 10, dir: 2)
+Number of deleted files: 3
+Number of regular files transferred: 8
+Total file size: 1.50M bytes
+Total transferred file size: 24.00K bytes
+Literal data: 24.00K bytes
+Matched data: 976.00K bytes
+File list size: 8.00K
+Total bytes sent: 10.00K
+Total bytes received: 2.00K
+sent 10.00K bytes  received 2.00K bytes  1.20K bytes/sec
+total size is 1.50M  speedup is 125.00
+"""
+        )
+
+        self.assertEqual(stats["files_total"], 1234)
+        self.assertEqual(stats["files_created"], 12)
+        self.assertEqual(stats["files_deleted"], 3)
+        self.assertEqual(stats["files_transferred"], 8)
+        self.assertEqual(stats["total_file_size_bytes"], 1_500_000)
+        self.assertEqual(stats["total_transferred_file_size_bytes"], 24_000)
+        self.assertEqual(stats["literal_data_bytes"], 24_000)
+        self.assertEqual(stats["matched_data_bytes"], 976_000)
+        self.assertEqual(stats["bytes_sent_received"], 12_000)
+        self.assertEqual(stats["bytes_per_second"], 1_200)
+        self.assertEqual(stats["speedup"], 125.0)
+        self.assertEqual(stats["link_dest_estimated_savings_bytes"], 976_000)
+        self.assertEqual(stats["link_dest_estimated_savings_ratio"], 0.976)
+
+    def test_tree_usage_reports_hardlinked_files(self) -> None:
+        with TemporaryDirectory() as tmp:
+            root = Path(tmp)
+            source = root / "source"
+            linked = root / "linked"
+            source.write_bytes(b"abc")
+            os.link(source, linked)
+
+            stats = tree_usage(root)
+
+        self.assertEqual(stats["files"], 2)
+        self.assertEqual(stats["apparent_size_bytes"], 6)
+        self.assertEqual(stats["hardlinked_files"], 2)
+        self.assertEqual(stats["hardlinked_apparent_size_bytes"], 6)
+        self.assertEqual(stats["hardlink_apparent_ratio"], 1.0)