feat: add article_history DB module (check, insert, cleanup)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/db/article_history.rs

@@ -0,0 +1,76 @@
+//! Article history: tracks which article URLs have been used in past syntheses.
+//!
+//! Prevents the same article from appearing in multiple syntheses.
+
+use std::collections::HashSet;
+use sqlx::PgPool;
+use uuid::Uuid;
+use crate::errors::AppError;
+
+/// Check which URL hashes already exist in history for this user.
+///
+/// Returns the set of url_hashes that were found (i.e., already used).
+pub async fn check_urls_exist(
+    pool: &PgPool,
+    user_id: Uuid,
+    url_hashes: &[String],
+) -> Result<HashSet<String>, AppError> {
+    if url_hashes.is_empty() {
+        return Ok(HashSet::new());
+    }
+
+    let rows = sqlx::query_scalar::<_, String>(
+        "SELECT url_hash FROM article_history WHERE user_id = $1 AND url_hash = ANY($2)",
+    )
+    .bind(user_id)
+    .bind(url_hashes)
+    .fetch_all(pool)
+    .await?;
+
+    Ok(rows.into_iter().collect())
+}
+
+/// Insert article URLs into history.
+///
+/// Uses ON CONFLICT DO NOTHING to silently skip duplicates.
+pub async fn insert_urls(
+    pool: &PgPool,
+    user_id: Uuid,
+    urls: &[(String, String)], // Vec<(url, url_hash)>
+) -> Result<(), AppError> {
+    if urls.is_empty() {
+        return Ok(());
+    }
+
+    for (url, url_hash) in urls {
+        sqlx::query(
+            "INSERT INTO article_history (user_id, url_hash, url) VALUES ($1, $2, $3) ON CONFLICT DO NOTHING",
+        )
+        .bind(user_id)
+        .bind(url_hash)
+        .bind(url)
+        .execute(pool)
+        .await?;
+    }
+
+    Ok(())
+}
+
+/// Delete history entries older than N days for this user.
+///
+/// Returns the number of deleted rows.
+pub async fn cleanup_old(
+    pool: &PgPool,
+    user_id: Uuid,
+    days: i32,
+) -> Result<u64, AppError> {
+    let result = sqlx::query(
+        "DELETE FROM article_history WHERE user_id = $1 AND created_at < now() - make_interval(days => $2)",
+    )
+    .bind(user_id)
+    .bind(days)
+    .execute(pool)
+    .await?;
+
+    Ok(result.rows_affected())
+}

backend/src/db/mod.rs

@@ -1,3 +1,4 @@
+pub mod article_history;
 pub mod api_keys;
 pub mod audit;
 pub mod magic_links;