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/database.py

@@ -1,3 +1,5 @@
+import os
+
 from sqlalchemy.ext.asyncio import AsyncEngine, AsyncSession, async_sessionmaker, create_async_engine
 from sqlalchemy import create_engine, Engine, event
 from sqlmodel import SQLModel
@@ -11,9 +13,20 @@ def get_async_engine(db_path: str) -> AsyncEngine:
     prefix = "sqlite+aiosqlite:///"
     if db_path.startswith(":memory:"):
         prefix = "sqlite+aiosqlite://"
+
+    pool_size = int(os.environ.get("DECNET_DB_POOL_SIZE", "20"))
+    max_overflow = int(os.environ.get("DECNET_DB_MAX_OVERFLOW", "40"))
+
+    pool_recycle = int(os.environ.get("DECNET_DB_POOL_RECYCLE", "3600"))
+    pool_pre_ping = os.environ.get("DECNET_DB_POOL_PRE_PING", "true").lower() == "true"
+
     engine = create_async_engine(
         f"{prefix}{db_path}",
         echo=False,
+        pool_size=pool_size,
+        max_overflow=max_overflow,
+        pool_recycle=pool_recycle,
+        pool_pre_ping=pool_pre_ping,
         connect_args={"uri": True, "timeout": 30},
     )
 

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()]