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/scanner.go

@@ -10,6 +10,20 @@ import (
 	"naviwatcher/internal/normalize"
 )
 
+// DefaultThreshold is the fallback similarity threshold used when a caller
+// passes threshold == 0. It matches config.Scanner.FuzzyThreshold default.
+const DefaultThreshold = 0.85
+
+// resolveThreshold returns the provided threshold, or DefaultThreshold when
+// the caller passes zero (unset). This keeps the engine usable when config
+// defaults are not threaded through explicitly.
+func resolveThreshold(threshold float64) float64 {
+	if threshold == 0 {
+		return DefaultThreshold
+	}
+	return threshold
+}
+
 // Similarity returns a normalized similarity score in the range [0.0, 1.0]
 // between two strings. The strings are normalized first (lowercased,
 // bracketed/parenthesized content and years stripped, special characters