feat(reactions): per-user like/dislike on mangas

Add a private per-user reaction resource: a manga_reactions table (one row
per user+manga, like/dislike, cascades) and endpoints PUT/DELETE
/mangas/:id/reaction plus GET /me/reactions/:manga_id, wired on the existing
per-user seam (mirrors bookmarks/read_progress). Unknown manga → 404 via the
FK-violation mapping; an invalid reaction value → 422. Reactions are never
exposed publicly — this is a taste signal for the upcoming content-based
recommendations. Integration tests cover set/toggle/clear/read, per-user
isolation, and the 404/422/401 paths.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/src/repo/mod.rs

@@ -13,6 +13,7 @@ pub mod manga;
 pub mod page;
 pub mod page_analysis;
 pub mod page_tag;
+pub mod reaction;
 pub mod read_progress;
 pub mod session;
 pub mod storage_stats;

backend/src/repo/reaction.rs

@@ -0,0 +1,66 @@
+//! Per-user manga reaction (like/dislike) persistence.
+
+use sqlx::PgPool;
+use uuid::Uuid;
+
+use crate::domain::reaction::Reaction;
+use crate::error::{AppError, AppResult};
+
+/// Insert-or-overwrite the user's reaction on this manga (like ↔ dislike).
+/// A foreign-key violation (unknown manga) maps to `NotFound` so the API
+/// returns 404 rather than 500 — mirrors `read_progress::upsert`.
+pub async fn upsert(
+    pool: &PgPool,
+    user_id: Uuid,
+    manga_id: Uuid,
+    reaction: Reaction,
+) -> AppResult<()> {
+    sqlx::query(
+        r#"
+        INSERT INTO manga_reactions (user_id, manga_id, reaction, created_at)
+        VALUES ($1, $2, $3, now())
+        ON CONFLICT (user_id, manga_id) DO UPDATE
+          SET reaction   = EXCLUDED.reaction,
+              created_at = now()
+        "#,
+    )
+    .bind(user_id)
+    .bind(manga_id)
+    .bind(reaction.as_str())
+    .execute(pool)
+    .await
+    .map_err(|e| match e {
+        sqlx::Error::Database(ref db_err) if db_err.is_foreign_key_violation() => {
+            AppError::NotFound
+        }
+        other => AppError::Database(other),
+    })?;
+    Ok(())
+}
+
+/// Remove the user's reaction on this manga. Idempotent — clearing a
+/// non-existent reaction is a no-op.
+pub async fn clear(pool: &PgPool, user_id: Uuid, manga_id: Uuid) -> AppResult<()> {
+    sqlx::query("DELETE FROM manga_reactions WHERE user_id = $1 AND manga_id = $2")
+        .bind(user_id)
+        .bind(manga_id)
+        .execute(pool)
+        .await?;
+    Ok(())
+}
+
+/// The user's current reaction on this manga, or `None` if unset.
+pub async fn get(
+    pool: &PgPool,
+    user_id: Uuid,
+    manga_id: Uuid,
+) -> AppResult<Option<Reaction>> {
+    let row: Option<(String,)> = sqlx::query_as(
+        "SELECT reaction FROM manga_reactions WHERE user_id = $1 AND manga_id = $2",
+    )
+    .bind(user_id)
+    .bind(manga_id)
+    .fetch_optional(pool)
+    .await?;
+    Ok(row.and_then(|(s,)| Reaction::parse(&s)))
+}