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/library_service.go

@@ -158,10 +158,39 @@ func (s *LibraryService) DeleteLibrary(ctx context.Context, libraryID pgtype.UUI
 }
 
 // AddLibraryFolder adds a folder to a library
+// Folder purposes (library_folders.purpose): 'scan' roots feed the FS
+// walker; 'podcast' roots are download targets written only by the
+// podcast download worker.
+const (
+	FolderPurposeScan    = "scan"
+	FolderPurposePodcast = "podcast"
+)
+
+// AddLibraryFolder registers a folder against a library. The purpose is
+// derived from the library type: podcasts libraries register their
+// download root (purpose='podcast', exactly ONE folder — feeds lay out
+// <download-root>/<feed-slug>/...); every other type registers a
+// scanner root (purpose='scan').
 func (s *LibraryService) AddLibraryFolder(ctx context.Context, libraryID pgtype.UUID, folderPath string) (*database.LibraryFolders, error) {
+	purpose := FolderPurposeScan
+
+	lib, err := s.db.GetLibrary(ctx, libraryID)
+	if err == nil && lib.TypeName == LibraryTypePodcasts {
+		// Podcasts libraries own exactly one folder: the download root.
+		existing, err := s.db.GetLibraryFolders(ctx, libraryID)
+		if err != nil {
+			return nil, err
+		}
+		if len(existing) > 0 {
+			return nil, fmt.Errorf("podcasts libraries have a single download root; %s is already registered", existing[0].FolderPath)
+		}
+		purpose = FolderPurposePodcast
+	}
+
 	folder, err := s.db.AddLibraryFolder(ctx, database.AddLibraryFolderParams{
 		LibraryID:  libraryID,
 		FolderPath: folderPath,
+		Purpose:    purpose,
 	})
 	if err != nil {
 		return nil, err