auth-server: durable (Postgres) persistence for sessions and clients

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Context

The open-sourced auths-auth-server (#317) ships with in-memory + git-backed stores only. The sqlx/Postgres adapters were dropped because sqlx pulls a native sqlite3 driver (libsqlite3-sys) that collides with auths-core's sqlite crate — two packages declaring links = "sqlite3", which cargo forbids.

The Postgres adapter source + migrations remain in the tree (migrations/), and the store ports (ports/session_store.rs, ports/client_store.rs) are storage-agnostic.

Task

Restore durable persistence without the native-lib conflict. Options:

• Make the Postgres path opt-in and stop sqlx pulling the sqlite driver (drop the macros/migrate features and use runtime sqlx::query, or otherwise ensure only sqlx-postgres is in the graph), OR
• Use a different Postgres client that doesn't link sqlite, OR
• Reconcile auths-core's sqlite dependency so both can coexist.

Acceptance

• cargo deny/build green with Postgres enabled (no duplicate links = "sqlite3").
• Session + client stores persist across restarts; covered by tests behind a postgres feature.