Parent epic: #2385

Problem

Several test modules use sqlite-specific APIs directly, preventing the test suite from running against PostgreSQL:

• crates/zeph-memory/src/graph/store/tests.rs:869 — use sqlx::sqlite::SqliteConnectOptions
• crates/zeph-index/src/watcher.rs:80 — sqlx::SqlitePool::connect("sqlite::memory:")
• crates/zeph-scheduler/src/scheduler.rs:377 — sqlx::SqlitePool::connect("sqlite::memory:")
• crates/zeph-memory/src/store/mod.rs:98,113 — sqlite-specific PRAGMAs in tests

Required Changes

1. Replace all sqlx::SqlitePool::connect("sqlite::memory:") with zeph_db::DbConfig { url: ":memory:".into(), .. }.connect().await
2. Replace SqliteConnectOptions usage with zeph-db pool creation
3. Gate sqlite-specific test assertions (PRAGMAs) behind #[cfg(feature = "sqlite")]
4. Add testcontainers-based postgres test support to consumer crates that need it
5. Add CI matrix: run workspace tests with both --features sqlite and --features postgres

Acceptance Criteria

• cargo nextest run --features postgres passes all non-ignored tests
• CI runs tests against both backends
• No sqlx::sqlite::* imports outside zeph-db