db: switch MySQL driver to asyncmy, env-tune pool, serialize DDL

- aiomysql → asyncmy on both sides of the URL/import (faster, maintained).
- Pool sizing now reads DECNET_DB_POOL_SIZE / MAX_OVERFLOW / RECYCLE /
  PRE_PING for both SQLite and MySQL engines so stress runs can bump
  without code edits.
- MySQL initialize() now wraps schema DDL in a GET_LOCK advisory lock so
  concurrent uvicorn workers racing create_all() don't hit 'Table was
  skipped since its definition is being modified by concurrent DDL'.
- sqlite & mysql repo get_log_histogram use the shared _session() helper
  instead of session_factory() for consistency with the rest of the repo.
- SSE stream_events docstring updated to asyncmy.

decnet/web/db/sqlite/repository.py

@@ -54,6 +54,6 @@ class SQLiteRepository(SQLModelRepository):
             literal_column("bucket_time")
         )
 
-        async with self.session_factory() as session:
+        async with self._session() as session:
             results = await session.execute(statement)
             return [{"time": r[0], "count": r[1]} for r in results.all()]