Persist trading state to Postgres with boot rehydrate

[ozpool]

What

Backs the edge's in-memory stores (orders, positions, fills, vault balances, public trade tape, funding history) with Postgres so a restart rehydrates instead of starting empty.

How

• Reads stay on the hot in-memory path — unchanged.
• Writes mirror through a channel: each mutator emits a PersistEvent; one background writer task drains it and runs the SQL in order. Keeps the sync, lock-guarded mutators off async I/O.
• Boot loads a full snapshot from Postgres, repopulates the maps, and rebuilds each market's orderbook from the rehydrated resting orders.
• Schema in crates/perplex-edge/migrations/0001_init.sql, applied idempotently on connect.

Derived/ephemeral state is deliberately not persisted: the orderbook is rebuilt from open_orders, index prices come from the live oracle, SIWE nonces expire in seconds.

The binary now requires DATABASE_URL (the bundled Postgres already ships in docker-compose.yml). Unit tests are untouched — AppState::new stays a pure in-memory path with no channel.

Test

• 31 edge unit/integration tests green, including new coverage for snapshot rehydration and event emission (no live DB needed).
• Manual end-to-end against local Postgres: placed a resting order and matched a taker against a maker, killed the process, restarted — orders, decremented maker remaining, both positions, fills, balances, and the rebuilt orderbook all survived.

Notes

• Durability has a small window: the write channel is in-memory, so a hard crash can drop the last few unflushed events. Acceptable at this stage; synchronous-commit durability would need the mutators made async.