Switch HashTable to shared_ptr and add missing returns

Use std::shared_ptr<Track> in HashTable to avoid raw pointer ownership
issues. Also add missing return statements in Database ID lookup helpers.

include/Containers.h

@@ -53,8 +53,8 @@ class HashTable {
     HashTable ( HashTable && ) noexcept = default;
     HashTable &operator= ( HashTable && ) noexcept = default;
 
-    void ht_insert ( const std::string &key, Track* t );
-    Track* ht_lookup ( const std::string &key ) const;
+    void ht_insert ( const std::string &key, std::shared_ptr<Track> t );
+    std::shared_ptr<Track> ht_lookup ( const std::string &key ) const;
     bool ht_delete ( const std::string &key );
 
     std::vector<std::string> listAll () const;
@@ -72,7 +72,7 @@ class HashTable {
     struct Entry {
         std::string key;
         uint64_t hash = 0;
-        Track* track;
+        std::shared_ptr<Track> track;
     };
 
     // helpers