docs: add database context manager to completed work

TODO.md

@@ -656,6 +656,13 @@ becomes a constraint.
 - Process memory from /proc/self/status
 - GC statistics and collection counts
 
+### [x] Database Context Manager Refactoring
+- Refactored all DB operations to use `_db_context()` context manager
+- `prepare_jobs()`, `submit_collected()`, `_run()` now use `with self._db_context() as db:`
+- `fetch_rows()` accepts db parameter for dependency injection
+- Removed deprecated `_prep_db()` and `_close_db()` methods
+- Connections guaranteed to close even on exceptions
+
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