feat(podcasts): scanner exclusion — podcasts libraries are feed-managed (task 1.2)

WHY: podcast download roots hold fetcher-written episode files; the FS
walker must never discover them (invariant 4) — a walk would double-
ingest episodes as scanner items and fight the fetcher's sole-writer
rule (D9). Every scan/watch path funnels through MediaScanner.SetFolders
(StartScanner, ScanLibrary jobs incl. scoped single-folder scans,
watch mode), so the filter lives there once: folders resolving to a
podcasts-type library are dropped before registration, with a log line.

Belt and braces: ScanLibrary's per-folder scope refuses purpose='podcast'
folders explicitly (400) — the panel-level path that bypasses type
resolution. AddLibraryFolder now derives purpose from the library type
('podcast' for podcasts libraries, enforced single download root;
'scan' default elsewhere), so folder purpose and library type cannot
diverge through the API.

internal/services/media_scanner.go

@@ -512,6 +512,33 @@ func (s *MediaScanner) GetStats() (int, int, int) {
 // watcher (and thus no fd/inotify watches) is allocated — the watcher is never
 // read by scan jobs and previously leaked one watcher per job.
 func (s *MediaScanner) SetFolders(folders []string, watch bool) error {
+	// Podcasts libraries are feed-managed (plan: podcast-integration.md
+	// task 1.2, invariant 4): their download roots are written by the
+	// download worker, never discovered by the walker — including
+	// scoped single-folder scans (0f1d0abb), which all funnel through
+	// here. Filter them out before anything else touches the list.
+	scannable := folders[:0:0]
+	for _, folder := range folders {
+		lib, err := s.db.GetLibraryByFolder(context.Background(), folder)
+		if err != nil {
+			// Unresolvable folders keep the old behavior (scan walk
+			// skips them per-file; watch adds them harmlessly).
+			scannable = append(scannable, folder)
+			continue
+		}
+		libType, err := s.db.GetLibraryType(context.Background(), lib.LibraryTypeID)
+		if err != nil {
+			scannable = append(scannable, folder)
+			continue
+		}
+		if libType.Name == LibraryTypePodcasts {
+			fmt.Printf("Scanner: skipping %s (podcasts library %q — feed-managed, not scanned)\n", folder, lib.Name)
+			continue
+		}
+		scannable = append(scannable, folder)
+	}
+	folders = scannable
+
 	s.folders = folders
 
 	// Always close any previously-owned watcher so reconfiguration doesn't leak.
@@ -524,6 +551,12 @@ func (s *MediaScanner) SetFolders(folders []string, watch bool) error {
 
 	// Create + populate a fresh watcher only when the caller intends to read events.
 	if watch {
+		if len(folders) == 0 {
+			// All requested folders filtered (e.g. a podcasts library:
+			// nothing scannable, nothing to watch). Fail loudly at the
+			// API boundary instead of panicking on folders[0] below.
+			return fmt.Errorf("no scannable folders (podcasts libraries are feed-managed and cannot be scanned)")
+		}
 		watcher, err := fsnotify.NewWatcher()
 		if err != nil {
 			// Return an error instead of panicking so a failed watcher can't