chore: Refactor stores to use PersistableStore CRTP template (#2464)

src/OpdsServerStore.h

@@ -1,4 +1,7 @@
 #pragma once
+#include <ArduinoJson.h>
+#include <PersistableStore.h>
+
 #include <string>
 #include <vector>
 
@@ -9,37 +12,25 @@ struct OpdsServer {
   std::string password;  // Plaintext in memory; obfuscated with hardware key on disk
 };
 
-class OpdsServerStore;
-namespace JsonSettingsIO {
-bool saveOpds(const OpdsServerStore& store, const char* path);
-bool loadOpds(OpdsServerStore& store, const char* json, bool* needsResave);
-}  // namespace JsonSettingsIO
-
 /**
  * Singleton class for storing OPDS server configurations on the SD card.
  * Passwords are XOR-obfuscated with the device's unique hardware MAC address
  * and base64-encoded before writing to JSON.
  */
-class OpdsServerStore {
+class OpdsServerStore : public PersistableStore<OpdsServerStore> {
  private:
-  static OpdsServerStore instance;
   std::vector<OpdsServer> servers;
 
   static constexpr size_t MAX_SERVERS = 8;
 
   OpdsServerStore() = default;
 
-  friend bool JsonSettingsIO::saveOpds(const OpdsServerStore&, const char*);
-  friend bool JsonSettingsIO::loadOpds(OpdsServerStore&, const char*, bool*);
+  friend class PersistableStore<OpdsServerStore>;
 
  public:
-  OpdsServerStore(const OpdsServerStore&) = delete;
-  OpdsServerStore& operator=(const OpdsServerStore&) = delete;
-
-  static OpdsServerStore& getInstance() { return instance; }
-
-  bool saveToFile() const;
-  bool loadFromFile();
+  static const char* getFilePath() { return "/.crosspoint/opds.json"; }
+  void toJson(JsonDocument& doc) const;
+  bool fromJson(JsonVariantConst doc);
 
   bool addServer(const OpdsServer& server);
   bool updateServer(size_t index, const OpdsServer& server);
@@ -49,12 +40,6 @@ class OpdsServerStore {
   const OpdsServer* getServer(size_t index) const;
   size_t getCount() const { return servers.size(); }
   bool hasServers() const { return !servers.empty(); }
-
-  /**
-   * Migrate from legacy single-server settings in CrossPointSettings.
-   * Called once during first load if no opds.json exists.
-   */
-  bool migrateFromSettings();
 };
 
 #define OPDS_STORE OpdsServerStore::getInstance()