Function update 

pub fn update(
    conn: &Connection,
    id: &str,
    title: Option<&str>,
    content: Option<&str>,
    tier: Option<&Tier>,
    namespace: Option<&str>,
    tags: Option<&Vec<String>>,
    priority: Option<i32>,
    confidence: Option<f64>,
    expires_at: Option<&str>,
    metadata: Option<&Value>,
) -> Result<(bool, bool)>

Update a memory by ID. Returns (found, content_changed) so callers can re-generate embeddings when the searchable text has changed.