feat(02-02): add ScanWarning type and reclassify scan errors as warnings

- Add ScanWarning struct with FilePath/Phase/Err to ScanMetrics
- Add mutex-protected addWarning method for concurrent use
- Reclassify walk, extraction, commit, orphan, variant, FTS failures as warnings
- Update commitBatch to return only fatal tx.Commit errors
- Update cachedLinkArtist to check errors via database.IsUniqueViolation
- Update handleConfigUpdate to capture and log scan warning count

backend/library/metrics.go

@@ -49,6 +49,16 @@ type ScanMetrics struct {
 	Updated int64 `json:"updated"`
 	Skipped int64 `json:"skipped"`
 	Removed int64 `json:"removed"`
+
+	// Non-fatal issues encountered during scanning.
+	Warnings []ScanWarning `json:"warnings"`
+}
+
+// ScanWarning represents a non-fatal issue encountered during scanning.
+type ScanWarning struct {
+	FilePath string `json:"filePath"`
+	Phase    string `json:"phase"`
+	Err      error  `json:"err"`
 }
 
 func newScanMetrics() *ScanMetrics {
@@ -80,6 +90,18 @@ func (m *ScanMetrics) addCoverArtSave(d time.Duration) {
 	m.CoverArtSave += d
 }
 
+// addWarning records a non-fatal scan issue.  Safe for concurrent use.
+func (m *ScanMetrics) addWarning(filePath, phase string, err error) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	m.Warnings = append(m.Warnings, ScanWarning{
+		FilePath: filePath,
+		Phase:    phase,
+		Err:      err,
+	})
+}
+
 // addThumbnailTier records the time spent generating a single
 // thumbnail tier.  Safe for concurrent use from the thumbnail
 // worker pool.