refactor: inject messaging avatar url builder

[shuxueshuxue]

Summary

• introduce messaging/avatars.py as the messaging-owned avatar URL protocol
• stop messaging/service.py and messaging/delivery/dispatcher.py from importing backend.web.utils.serializers directly
• bind the real file-backed avatar URL builder from backend/web/core/lifespan.py, while keeping missing builder usage fail-loud on actual avatar projection paths

Verification

• uv run python -m pytest tests/Unit/messaging/test_chat_delivery_dispatcher.py tests/Unit/messaging/test_module_independence.py tests/Integration/test_messaging_social_handle_contract.py -q -k "avatar or messaging_service or deliver_to_agents or chat_tool_list_chats or same_owner_agent_turn_delivers_to_sibling_actor_without_relationship"
• uv run ruff check messaging/avatars.py messaging/service.py messaging/delivery/dispatcher.py backend/web/core/lifespan.py tests/Unit/messaging/test_chat_delivery_dispatcher.py tests/Unit/messaging/test_module_independence.py tests/Integration/test_messaging_social_handle_contract.py
• .venv/bin/python -m pyright messaging/avatars.py messaging/service.py messaging/delivery/dispatcher.py
• git diff --check