Implement SQLite conversation store with sqlx

[bug-ops]

Description

Persistent storage for conversations and messages using SQLite via sqlx.

Parent: #4

Branch

feat/m3/sqlite-memory

Files

• crates/zeph-memory/src/sqlite.rs — SqliteStore struct
• crates/zeph-memory/src/lib.rs — exports
• crates/zeph-memory/Cargo.toml — add sqlx (sqlite, runtime-tokio)
• migrations/001_init.sql — DDL

Schema

CREATE TABLE conversations (
    id INTEGER PRIMARY KEY AUTOINCREMENT,
    created_at TEXT NOT NULL DEFAULT (datetime('now'))
);

CREATE TABLE messages (
    id INTEGER PRIMARY KEY AUTOINCREMENT,
    conversation_id INTEGER NOT NULL REFERENCES conversations(id),
    role TEXT NOT NULL,
    content TEXT NOT NULL,
    created_at TEXT NOT NULL DEFAULT (datetime('now'))
);

Acceptance Criteria

• Creates database file if not exists
• Migrations run on startup
• CRUD operations work
• Unit tests with in-memory SQLite (:memory:)