feat: add DB-backed scanner entrypoint and compute-only main hook

Implements ScanArtist/ScanAll in internal/scanner loading local/external
releases via the database layer with ctx-cancellation checks, plus a
compute-only run() hook that logs missing-release counts. Add table-driven
tests using in-memory SQLite fixtures.

internal/scanner/scan.go

@@ -0,0 +1,66 @@
+package scanner
+
+import (
+	"context"
+
+	"naviwatcher/internal/database"
+)
+
+// ScanArtist loads the local albums and external releases for a single artist
+// from the database and computes the list of missing releases.
+//
+// threshold is the fuzzy-similarity cutoff; pass 0 to use DefaultThreshold.
+// The context is checked before querying the database; if it is already
+// cancelled, no work is performed and the sentinel error ctx.Err() is returned.
+func ScanArtist(ctx context.Context, db *database.DB, artistID string, threshold float64) ([]MissingRelease, error) {
+	if err := ctx.Err(); err != nil {
+		return nil, err
+	}
+
+	local, err := database.GetLocalAlbumsByArtist(db, artistID)
+	if err != nil {
+		return nil, err
+	}
+
+	external, err := database.GetExternalReleasesByArtist(db, artistID)
+	if err != nil {
+		return nil, err
+	}
+
+	missing := FindMissingReleases(local, external, resolveThreshold(threshold))
+	return missing, nil
+}
+
+// ScanAll iterates over all monitored artists (those with Monitored == true)
+// and computes the missing releases for each. Results are concatenated into a
+// single slice across all artists.
+//
+// ctx.Err() is checked between artists; if cancellation occurs mid-iteration,
+// scanning stops early and the accumulated results so far are returned along
+// with the cancellation error. threshold follows the same contract as
+// ScanArtist (0 → DefaultThreshold).
+func ScanAll(ctx context.Context, db *database.DB, threshold float64) ([]MissingRelease, error) {
+	settings, err := database.GetAllArtistSettings(db)
+	if err != nil {
+		return nil, err
+	}
+
+	resolved := resolveThreshold(threshold)
+
+	var all []MissingRelease
+	for _, s := range settings {
+		if err := ctx.Err(); err != nil {
+			return all, err
+		}
+		if !s.Monitored {
+			continue
+		}
+		missing, err := ScanArtist(ctx, db, s.ID, resolved)
+		if err != nil {
+			return all, err
+		}
+		all = append(all, missing...)
+	}
+
+	return all, nil
+}