Remove Tasks.md spec, add test for _cleanup_wal_files skipping recent files

Remove 335-line task specification from Docs/Tasks.md.
Add test confirming _cleanup_wal_files skips recently-modified WAL/SHM files.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/tests/test_db.py

@@ -252,6 +252,30 @@ async def test_cleanup_wal_files_removes_orphaned_files(tmp_path: Path) -> None:
     assert not shm_path.exists()
 
 
+async def test_cleanup_wal_files_skips_recent_files(tmp_path: Path) -> None:
+    """Test that _cleanup_wal_files skips files modified within 10 seconds."""
+    db_path = str(tmp_path / "test_wal_recent.db")
+    wal_path = Path(db_path + "-wal")
+    shm_path = Path(db_path + "-shm")
+
+    # Create files with recent mtime
+    wal_path.write_text("recent")
+    shm_path.write_text("recent")
+    recent_mtime = time.time() - 5
+    os.utime(wal_path, (recent_mtime, recent_mtime))
+    os.utime(shm_path, (recent_mtime, recent_mtime))
+
+    assert wal_path.exists()
+    assert shm_path.exists()
+
+    # Run cleanup
+    await _cleanup_wal_files(db_path)
+
+    # Files should NOT be removed (recent)
+    assert wal_path.exists()
+    assert shm_path.exists()
+
+
 async def test_cleanup_wal_files_handles_missing_files(tmp_path: Path) -> None:
     """Test that _cleanup_wal_files handles non-existent files gracefully."""
     db_path = str(tmp_path / "nonexistent.db")