feat(db): implement WAL mode for SQLite to improve concurrency and error handling

Author: ImMohammad20000

app/db/base.py

@@ -1,6 +1,6 @@
 from typing import Any
 
-from sqlalchemy import MetaData
+from sqlalchemy import MetaData, event
 from sqlalchemy.ext.asyncio import AsyncAttrs, async_sessionmaker, create_async_engine
 from sqlalchemy.orm import DeclarativeBase, MappedAsDataclass
 
@@ -33,7 +33,7 @@ def build_engine_options(settings: DatabaseSettings) -> dict[str, Any]:
 def database_pool_summary(settings: DatabaseSettings, process_count: int = 1) -> str:
     """Describe the effective connection budget without exposing the database URL."""
     if settings.is_sqlite:
-        return "Database pool: SQLite driver-managed connections (QueuePool settings do not apply)."
+        return "Database pool: SQLite driver-managed connections in WAL mode (QueuePool settings do not apply)."
 
     per_process = settings.connection_ceiling()
     service_ceiling = settings.connection_ceiling(process_count)
@@ -47,6 +47,18 @@ def database_pool_summary(settings: DatabaseSettings, process_count: int = 1) ->
 
 engine = create_async_engine(database_settings.url, **build_engine_options(database_settings))
 
+if database_settings.is_sqlite:
+    # WAL mode lets readers proceed concurrently with the single writer, preventing
+    # "database is locked" errors under async workloads. busy_timeout makes SQLite
+    # retry for up to 5 s on a locked write instead of failing immediately.
+    @event.listens_for(engine.sync_engine, "connect")
+    def _set_sqlite_pragmas(dbapi_conn, _connection_record):
+        cursor = dbapi_conn.cursor()
+        cursor.execute("PRAGMA journal_mode=WAL")
+        cursor.execute("PRAGMA busy_timeout=5000")
+        cursor.close()
+
+
 SessionLocal = async_sessionmaker(autocommit=False, autoflush=False, expire_on_commit=False, bind=engine)
 
 naming_convention = {