Storage: add durable PostgreSQL drift store

[BadgerOps]

Follow-up to #187 and PR #188.

Summary

Implement a PostgreSQL-backed drift store so drift items and computed network conflict decisions are durable in Postgres deployments.

Why

PR #188 persists computed conflict decisions through the configured drift store. SQLite is durable, but the current PostgreSQL wiring still relies on the existing in-memory drift fallback.

Scope

• Add Postgres migrations for drift_items equivalent to the SQLite schema.
• Implement storage.DriftStore for Postgres.
• Wire Postgres builds to use the durable drift store.
• Add tests for create/get/list/update/delete behavior.

Acceptance Criteria

• Postgres deployments retain drift and computed conflict decisions across process restarts.
• Existing drift API behavior matches SQLite and memory implementations.
• Postgres race tests cover the drift store path.