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/domain/mod.rs

@@ -11,6 +11,7 @@ pub mod page;
 pub mod page_analysis;
 pub mod page_tag;
 pub mod patch;
+pub mod reaction;
 pub mod read_progress;
 pub mod session;
 pub mod storage_stats;

backend/src/domain/reaction.rs

@@ -0,0 +1,40 @@
+use serde::{Deserialize, Serialize};
+use uuid::Uuid;
+
+/// A user's private taste signal on a manga. Stored as text in
+/// `manga_reactions.reaction` (CHECK-constrained), so we map to/from a
+/// `&str` at the repo layer rather than deriving a Postgres enum type.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "lowercase")]
+pub enum Reaction {
+    Like,
+    Dislike,
+}
+
+impl Reaction {
+    pub fn as_str(self) -> &'static str {
+        match self {
+            Reaction::Like => "like",
+            Reaction::Dislike => "dislike",
+        }
+    }
+
+    /// Parse a stored/inbound value. Returns `None` for anything outside the
+    /// closed vocabulary (the DB CHECK guarantees stored rows are valid; this
+    /// also guards the inbound API body).
+    pub fn parse(s: &str) -> Option<Reaction> {
+        match s {
+            "like" => Some(Reaction::Like),
+            "dislike" => Some(Reaction::Dislike),
+            _ => None,
+        }
+    }
+}
+
+/// Response shape for `GET /me/reactions/:manga_id` — the current user's
+/// reaction on one manga, or `null` when they haven't reacted.
+#[derive(Debug, Clone, Serialize)]
+pub struct MangaReaction {
+    pub manga_id: Uuid,
+    pub reaction: Option<Reaction>,
+}