feat(api-rs): persist session personas

[goksu]

Summary:

• add persona_id to CreateSessionRequest and Session
• persist persona_id through SQLx and reject mismatched session reuse with 409
• add SQLx migration 0006_session_persona_id.sql

Tests:

• rustfmt --edition 2024 --check on edited Rust files
• cargo test -p centaur-api-server -p centaur-session-runtime -p centaur-session-sqlx -p centaur-session-cli
• just build-one api-rs
• local Helm deploy with iron-control disabled for arm64 validation
• POST /api/session/web:persona-session-validation-1 returned persona_id=validation
• repeat POST with a different persona returned HTTP 409