feat(autotag): name the confidence tier two features have to share

`ConfidentTier` and `Confident()` are a name for what was about to be
written as `== RecommendationStrong` at two call sites: the album page
telling the user unprompted that there is a match for what they are
looking at (#28), and strict auto-accept rewriting files without asking
(#90). A page that claims confidence the auto-accept pass would decline
is the app contradicting itself, and #90 asks for exactly this — that
the two agree on what "high confidence" means rather than computing it
twice.

What they do not share is written down beside it. Surfacing a match is
a suggestion with a confirm dialog behind it; auto-accept is an
irreversible on-disk rewrite gated on further conditions the tier
cannot express — exact track count, every title matching, lengths
within a couple of seconds, no cover replacement, no MBID conflict. So
this is the floor both stand on, not the whole of either test.

`Confident` is a rank comparison rather than an equality, so a tier
added above "strong" later does not silently stop qualifying.

backend/autotag/recommend.go

@@ -17,6 +17,34 @@ const (
 	RecommendationStrong Recommendation = "strong"
 )
 
+// ConfidentTier is the tier at which this package considers a match
+// good enough to act on without being asked to look.
+//
+// It exists as a name rather than as `== RecommendationStrong` at
+// each call site because two features read it and they must not
+// disagree about what "high confidence" means: the album page tells
+// the user unprompted that the autotagger has a match (#28), and
+// strict auto-accept will rewrite the files without asking (#90).
+// A page that says "we are sure" about something the auto-accept
+// pass would decline is the app contradicting itself.
+//
+// What the two do *not* share is everything else. Surfacing a match
+// is a suggestion with a confirm dialog behind it; auto-accept is an
+// irreversible on-disk rewrite, and #90 gates it on further
+// conditions this tier cannot express — exact track count, every
+// title matching, lengths within a couple of seconds, no cover
+// replacement, no MBID conflict. So this is the floor both stand on,
+// not the whole of either test.
+const ConfidentTier = RecommendationStrong
+
+// Confident reports whether a tier clears ConfidentTier.
+//
+// A comparison rather than an equality, so adding a tier above
+// "strong" later does not silently stop qualifying.
+func Confident(r Recommendation) bool {
+	return recommendationRank(r) >= recommendationRank(ConfidentTier)
+}
+
 const (
 	// Absolute score tiers.
 	strongScoreThresh = 0.90

backend/autotag/recommend_test.go

@@ -168,3 +168,34 @@ func TestRecommend_LocalCandidatesWithoutRGMBIDCompareByTitle(t *testing.T) {
 		t.Errorf("different-title rival: Recommend = %q, want medium", got)
 	}
 }
+
+// The tier both features stand on is one name, checked here rather
+// than assumed at two call sites.
+//
+// #28 renders "we have a match for this album" on the album page and
+// #90 will rewrite files without asking; a page that claims confidence
+// the auto-accept pass would decline is the app contradicting itself.
+// What they do not share is everything else — auto-accept adds gates
+// this tier cannot express — so this pins the floor, not the whole of
+// either test.
+func TestConfidentIsTheOneSharedFloor(t *testing.T) {
+	t.Parallel()
+
+	if ConfidentTier != RecommendationStrong {
+		t.Errorf("ConfidentTier = %q, want strong", ConfidentTier)
+	}
+
+	for _, tc := range []struct {
+		rec  Recommendation
+		want bool
+	}{
+		{RecommendationNone, false},
+		{RecommendationLow, false},
+		{RecommendationMedium, false},
+		{RecommendationStrong, true},
+	} {
+		if got := Confident(tc.rec); got != tc.want {
+			t.Errorf("Confident(%q) = %v, want %v", tc.rec, got, tc.want)
+		}
+	}
+}