Studio Inbox: persist conversations + actions, with live handoff

[rahulsainlll]

Build the Analyze/Inbox back office for Studio so a product owner can see everything their agent does.

Scope

• New tables product_conversations + product_messages (+ action-run records), mirroring existing schema conventions.
• Persist widget conversations, messages, and action runs (with txHash where applicable).
• /studio/inbox UI (owner-scoped): list conversations per product, view the transcript, see actions run with on-chain proof links, and support human handoff (jump into a live conversation).

Acceptance

• Conversations + messages persist for both the embedded widget and the preview.
• Inbox lists them per product, owner-scoped (a user only sees their own).
• Action runs show a stellar.expert link.