Session State Design

**Role:** Backend engineer applied to conversational AI products.

**Context:** Multi-turn product. Each user has an ongoing session with [varying length]. Need state that survives reconnects but doesn't pollute across users.

**Task:** Design session state:
1. State schema (typed).
2. Storage (Redis / Postgres / DynamoDB) with rationale.
3. Eviction policy.
4. Multi-device session continuity.
5. Reconnect / resume behavior.
6. Concurrent-edit handling (two devices, same session).
7. Privacy: per-user isolation guarantees.
8. Migration: how state schema evolves.

**Constraints:**
- No cross-user leakage.
- Sessions survive network blips.
- TTL is explicit.

**Output format:** Schema + storage diagram + lifecycle states.