refactor(doctor): reuse backup's size formatter for database listings

_format_db_size reimplemented human-readable size formatting two
imports away from backup._format_size, which doctor already leans on
for _QUICK_STATE_FILES. Delegate and keep only the stat-failure wrap.
Sizes now scale units (KB/GB) instead of pinning everything to MB.

hermes_cli/doctor.py

@@ -142,11 +142,15 @@ def _read_journal_mode(db_path: Path) -> tuple[str | None, str | None]:
 
 
 def _format_db_size(db_path: Path) -> str:
+    # backup.py owns human-readable size formatting; reuse it (as with
+    # _QUICK_STATE_FILES above) and keep only the stat-failure wrap here.
+    from hermes_cli.backup import _format_size
+
     try:
-        mb = db_path.stat().st_size / 1_048_576
+        nbytes = db_path.stat().st_size
     except OSError:
         return "size unknown"
-    return f"{mb:.1f} MB" if mb >= 0.1 else "<0.1 MB"
+    return _format_size(nbytes)
 
 
 def _report_database_journal_modes(

tests/hermes_cli/test_doctor_journal_modes.py

@@ -8,6 +8,7 @@ read-only engine open creates -wal/-shm sidecar files next to a WAL database.
 """
 
 import os
+import re
 import sqlite3
 
 import pytest
@@ -257,7 +258,8 @@ class TestSizeAndRepairHint:
         _make_db(db, journal_mode="WAL")
         doctor._report_database_journal_modes(tmp_path, VULNERABLE)
         out = capsys.readouterr().out
-        assert "MB)" in out
+        # _format_size picks the unit (a fresh test DB is KB-scale).
+        assert re.search(r"\(\d[\d.]* [KMGT]?B\)", out)
         assert "To clear the exposure:" in out
 
     def test_no_repair_hint_when_nothing_is_exposed(self, tmp_path, capsys):