feat: add recommendation scoring and reranking engine

internal/anime/recommendations_test.go

@@ -0,0 +1,73 @@
+package anime
+
+import (
+	"database/sql"
+	"mal/integrations/jikan"
+	"mal/internal/db"
+	"testing"
+	"time"
+)
+
+func TestRecommendationEntryWeightPrioritizesCommittedRecentHistory(t *testing.T) {
+	now := time.Date(2026, time.June, 4, 12, 0, 0, 0, time.UTC)
+
+	completed := recommendationEntryWeight(now, db.GetUserWatchListRow{
+		Status:         "completed",
+		UpdatedAt:      now.Add(-24 * time.Hour),
+		CurrentEpisode: sql.NullInt64{Int64: 12, Valid: true},
+	})
+	planned := recommendationEntryWeight(now, db.GetUserWatchListRow{
+		Status:    "plan_to_watch",
+		UpdatedAt: now.Add(-24 * time.Hour),
+	})
+
+	if completed <= planned {
+		t.Fatalf("expected completed history to outrank planned history, got completed=%f planned=%f", completed, planned)
+	}
+}
+
+func TestBuildRecommendationSeedsFiltersUnsupportedStatuses(t *testing.T) {
+	now := time.Date(2026, time.June, 4, 12, 0, 0, 0, time.UTC)
+
+	seeds := buildRecommendationSeeds(now, []db.GetUserWatchListRow{
+		{AnimeID: 1, Status: "dropped", UpdatedAt: now},
+		{AnimeID: 2, Status: "watching", UpdatedAt: now},
+		{AnimeID: 3, Status: "completed", UpdatedAt: now},
+	})
+
+	if len(seeds) != 2 {
+		t.Fatalf("expected 2 valid seeds, got %d", len(seeds))
+	}
+	if seeds[0].animeID != 2 || seeds[1].animeID != 3 {
+		t.Fatalf("unexpected seed ordering: %+v", seeds)
+	}
+}
+
+func TestScoreRecommendationCandidateRewardsProfileOverlap(t *testing.T) {
+	now := time.Date(2026, time.June, 4, 12, 0, 0, 0, time.UTC)
+	profile := userTasteProfile{
+		genres: map[int]float64{
+			1: 2.0,
+		},
+		themes:       map[int]float64{},
+		studios:      map[int]float64{},
+		demographics: map[int]float64{},
+	}
+
+	matching := scoreRecommendationCandidate(now, profile, jikan.Anime{
+		MalID:      10,
+		Genres:     []jikan.NamedEntity{{MalID: 1, Name: "Action"}},
+		Popularity: 100,
+		Score:      8.0,
+	}, 5.0)
+	nonMatching := scoreRecommendationCandidate(now, profile, jikan.Anime{
+		MalID:      11,
+		Genres:     []jikan.NamedEntity{{MalID: 2, Name: "Drama"}},
+		Popularity: 100,
+		Score:      8.0,
+	}, 5.0)
+
+	if matching.score <= nonMatching.score {
+		t.Fatalf("expected matching candidate to score higher, got matching=%f nonMatching=%f", matching.score, nonMatching.score)
+	}
+}