feat: analytics backend API with chat_message table

- Add chat_message table for message-level analytics with usage JSON field
- Add migration to backfill from existing chats
- Add /analytics endpoints: summary, models, users, daily
- Support hourly/daily granularity for time-series data
- Fill missing days/hours in date range

backend/open_webui/main.py

@@ -68,6 +68,7 @@ from open_webui.socket.main import (
     get_models_in_use,
 )
 from open_webui.routers import (
+    analytics,
     audio,
     images,
     ollama,
@@ -1455,6 +1456,9 @@ app.include_router(functions.router, prefix="/api/v1/functions", tags=["function
 app.include_router(
     evaluations.router, prefix="/api/v1/evaluations", tags=["evaluations"]
 )
+app.include_router(
+    analytics.router, prefix="/api/v1/analytics", tags=["analytics"]
+)
 app.include_router(utils.router, prefix="/api/v1/utils", tags=["utils"])
 
 # SCIM 2.0 API for identity management