feat(session): round-trip streamable, parental_warning, source_item_id_hint (#315)

The PersistedQueueTrack struct already had streamable and parental_warning
fields but the load path hardcoded streamable=true and parental_warning
=false because the columns didn't exist in queue_tracks. source_item_id
_hint was missing entirely.

Adds the three columns via the same idempotent ALTER-TABLE pattern the
existing migrations use, persists them in save_session, and reads them in
load_session. PersistedQueueTrack gains source_item_id_hint as an
Option<String> with serde default so saves from older builds deserialize
cleanly.

User-visible effect: a restored queue now keeps explicit-content badges,
unstreamable visual state, and Mixtape/Collection source-item ids
matching the live queue instead of resetting to defaults at every
launch. Backwards-compatible: columns get safe defaults for older rows
(streamable=1, parental_warning=0, hint=NULL) so users upgrading from
1.2.12 don't see a regression.

cc

src/lib/services/sessionService.ts

@@ -26,6 +26,12 @@ export interface PersistedQueueTrack {
   // v2_play_track (Qobuz) instead of v2_library_play_track. Backend also
   // keeps this in session_store; this interface just needs to carry it.
   source?: string | null;
+  // Round-tripped fields, previously hardcoded on load. Persisting them
+  // means a restored queue matches the live one for explicit-content
+  // badges, unstreamable visual state, and Mixtape source association.
+  streamable?: boolean;
+  parental_warning?: boolean;
+  source_item_id_hint?: string | null;
 }
 
 export interface PersistedSession {

src/routes/+page.svelte

@@ -4200,6 +4200,9 @@
             // older versions that didn't persist source. Without this, a
             // restored local queue routed next-track auto-advance to Qobuz.
             source: trk.source ?? (trk.is_local ? 'local' : undefined),
+            streamable: trk.streamable ?? true,
+            parental_warning: trk.parental_warning ?? false,
+            source_item_id_hint: trk.source_item_id_hint ?? null,
           }));
 
           await setQueue(tracks, session.current_index ?? 0, true);
@@ -4479,6 +4482,13 @@
           // auto-advance routed to v2_play_track with library row ids — which
           // Qobuz then "resolved" to whatever track happened to share the id.
           source: track.source ?? (track.is_local ? 'local' : null),
+          // Round-trip explicit-content + unstreamable visual state + the
+          // Mixtape/Collection source-item id so a restored queue mirrors
+          // the live one. Backend defaults streamable=true, parental=false
+          // and source_item_id_hint=null for tracks saved by older builds.
+          streamable: track.streamable ?? true,
+          parental_warning: track.parental_warning ?? false,
+          source_item_id_hint: track.source_item_id_hint ?? null,
         });
       }