Enable SQLite WAL mode and SYNCHRONOUS=NORMAL

[bug-ops]

Parent: #621

Problem

crates/zeph-memory/src/sqlite/mod.rs:38-45 — SQLite defaults to DELETE journal mode. Every write acquires exclusive lock, blocking concurrent readers. Pool size is 5.

Solution

let opts = SqliteConnectOptions::from_str(&url)?
    .create_if_missing(true)
    .foreign_keys(true)
    .journal_mode(sqlx::sqlite::SqliteJournalMode::Wal)
    .synchronous(sqlx::sqlite::SqliteSynchronous::Normal);

Impact: 2-5x write throughput improvement with near-full durability.