php: fetch: prices: new functionality / refactor

- Add support for multiple aggregator APIs
- Refactor `prices` API implementation
- Update documentation

container/src/finance/lib/internal/fetch/prices/fetch.php

@@ -40,7 +40,7 @@ namespace docker_finance\prices
      */
     final class Fetch extends API
     {
-        private internal\prices\Crypto $api;  //!< Internal API
+        private mixed $api;  //!< Internal API
 
         public function __construct(utils\Env $env)
         {
@@ -50,14 +50,17 @@ namespace docker_finance\prices
         //! @brief Fetch executor
         public function fetch(): void
         {
-            $subtype = $this->get_env()->get_env('API_FETCH_SUBTYPE');
-            switch ($subtype) {
-                case 'crypto':
-                    $this->api = new internal\prices\Crypto($this->get_env());
+            $upstream = $this->get_env()->get_env('API_PRICES_API');
+            switch ($upstream) {
+                case 'coingecko':
+                    $this->api = new internal\prices\CoinGecko($this->get_env());
+                    break;
+                case 'mobula':
+                    $this->api = new internal\prices\Mobula($this->get_env());
                     break;
                 default:
                     utils\CLI::throw_fatal(
-                        "unsupported subtype '$subtype' for interal API"
+                        "unsupported upstream API '{$upstream}' for interal API"
                     );
                     break;
             }