feat: Session TTL management, periodic cleanup

[sihyeonn]

Handling #16219.

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Changelog Entry

Description

This PR migrates Session Survival to a pure TTL-based approach that works in both Redis and local modes.

• InMemoryTTLStore – lightweight in-memory replacement for Redis SETEX; async-friendly and auto-expires keys.
• SESSION_EXPIRY_POOL – single abstraction used by the socket layer; points to Redis or InMemoryTTLStore depending on WEBSOCKET_MANAGER.
• periodic_session_ttl_cleanup – background task started during FastAPI startup; removes stale sessions and keeps SESSION_POOL / USER_POOL in sync.
• Refactors & clean-ups – simplified disconnect logic, removed the unused MemorySetexManager alias.
• Tests – added unit tests for the new store and for the periodic cleanup loop.

Added

• open_webui.socket.utils.InMemoryTTLStore
• open_webui.socket.main.periodic_session_ttl_cleanup
• Tests
  • backend/open_webui/socket/test_utils.py
  • backend/open_webui/socket/test_main.py

Changed

• socket/main.py – uses SESSION_EXPIRY_POOL; startup scheduling on FastAPI lifespan.
• socket/utils.py – new store implementation, background thread removed.
• backend/open_webui/main.py – schedules the new periodic cleanup task.

Deprecated

• [List any deprecated functionality or features that have been removed]

Removed

• [List any removed features, files, or functionalities]

Fixed

• Stale sessions were not purged in local mode.
• Potential SESSION_POOL / USER_POOL inconsistencies in Redis mode.

Security

• [List any new or updated security-related changes, including vulnerability fixes]

Breaking Changes

• BREAKING CHANGE: [List any breaking changes affecting compatibility or functionality]

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Additional Information

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  • [Reference any related issues, commits, or other relevant information]

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