feat: implement scanner diff engine (missing-release detection)

Add FindMissingReleases and MissingRelease to internal/scanner: skips
IsIgnored external releases, scopes matches per ArtistID, and reports
external releases with no local album above the fuzzy threshold.

internal/scanner/diff.go

@@ -0,0 +1,63 @@
+package scanner
+
+import (
+	"naviwatcher/internal/database"
+)
+
+// MissingRelease describes an external release that has no sufficiently similar
+// local album. It is a flattened, consumer-friendly projection of a
+// database.ExternalRelease.
+type MissingRelease struct {
+	RGID        string `json:"rgid"`
+	ArtistID    string `json:"artist_id"`
+	Title       string `json:"title"`
+	Type        string `json:"type"`
+	ReleaseDate string `json:"release_date"`
+}
+
+// FindMissingReleases compares an artist's external discography against the
+// user's local albums and returns the releases that are present externally but
+// have no sufficiently similar local album.
+//
+// Rules:
+//   - External releases flagged IsIgnored are never reported.
+//   - A local album only matches an external release for the same ArtistID.
+//   - An external release is "missing" when none of the local albums (same
+//     ArtistID) IsMatch at the given threshold.
+func FindMissingReleases(local []database.LocalAlbum, external []database.ExternalRelease, threshold float64) []MissingRelease {
+	// Group local albums by artist for O(1) lookup per external release.
+	localByArtist := make(map[string][]database.LocalAlbum)
+	for _, a := range local {
+		localByArtist[a.ArtistID] = append(localByArtist[a.ArtistID], a)
+	}
+
+	var missing []MissingRelease
+	for _, ext := range external {
+		if ext.IsIgnored {
+			continue
+		}
+
+		albums := localByArtist[ext.ArtistID]
+		matched := false
+		for _, a := range albums {
+			if IsMatch(a.Title, ext.Title, threshold) {
+				matched = true
+				break
+			}
+		}
+
+		if matched {
+			continue
+		}
+
+		missing = append(missing, MissingRelease{
+			RGID:        ext.RGID,
+			ArtistID:    ext.ArtistID,
+			Title:       ext.Title,
+			Type:        ext.Type,
+			ReleaseDate: ext.ReleaseDate,
+		})
+	}
+
+	return missing
+}