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/router/stream/api_stream_events.py

@@ -66,7 +66,7 @@ async def stream_events(
 ) -> StreamingResponse:
 
     # Prefetch the initial snapshot before entering the streaming generator.
-    # With aiomysql (pure async TCP I/O), the first DB await inside the generator
+    # With asyncmy (pure async TCP I/O), the first DB await inside the generator
     # fires immediately after the ASGI layer sends the keepalive chunk — the HTTP
     # write and the MySQL read compete for asyncio I/O callbacks and the MySQL
     # callback can stall.  Running these here (normal async context, no streaming)