refactor(chat-store): extract message storage responsibility

[Andiveli]

Summary

• Extract message/chat storage from the chat-store coordinator.
• Preserve existing ingestion and WhatsApp event behavior.

Changes

• Add src/app/chat_store/storage.rs with focused boundary coverage.
• Keep existing App methods and delegate ownership through the child module.

Testing

• Focused storage boundary test
• cargo test app::chat_store --lib
• cargo test app::whatsapp_events --lib
• cargo test app::message_ingestion --lib
• git diff --check

Chain Context

beta
└── refactor/chat-store-srp (tracker)
    └── refactor/chat-store-hydration (#363)
        └── 📍 refactor/chat-store-storage

Start: hydration extraction.
End: message/chat storage has a dedicated owner.
Follow-up: isolate read-state and receipt coordination.
Out of scope: behavior, persistence schema, UI, and bridge changes.

Closes #361