Replace asyncio.to_thread with ThreadPoolExecutor.run_in_executor

- Add ThreadPoolExecutor with 3 max workers to SeriesApp
- Replace all asyncio.to_thread calls with loop.run_in_executor
- Add shutdown() method to properly cleanup executor
- Integrate SeriesApp.shutdown() into FastAPI shutdown sequence
- Ensures proper resource cleanup on Ctrl+C (SIGINT/SIGTERM)

src/core/SeriesApp.py

@@ -12,6 +12,7 @@ Note:
 
 import asyncio
 import logging
+from concurrent.futures import ThreadPoolExecutor
 from typing import Any, Dict, List, Optional
 
 from events import Events
@@ -148,6 +149,9 @@ class SeriesApp:
 
         self.directory_to_search = directory_to_search
 
+        # Initialize thread pool executor
+        self.executor = ThreadPoolExecutor(max_workers=3)
+
         # Initialize events
         self._events = Events()
         self._events.download_status = None
@@ -229,7 +233,9 @@ class SeriesApp:
 
     async def _init_list(self) -> None:
         """Initialize the series list with missing episodes (async)."""
-        self.series_list = await asyncio.to_thread(
+        loop = asyncio.get_running_loop()
+        self.series_list = await loop.run_in_executor(
+            self.executor,
             self.list.GetMissingEpisode
         )
         logger.debug(
@@ -251,7 +257,12 @@ class SeriesApp:
             RuntimeError: If search fails
         """
         logger.info("Searching for: %s", words)
-        results = await asyncio.to_thread(self.loader.search, words)
+        loop = asyncio.get_running_loop()
+        results = await loop.run_in_executor(
+            self.executor,
+            self.loader.search,
+            words
+        )
         logger.info("Found %d results", len(results))
         return results
       
@@ -348,7 +359,9 @@ class SeriesApp:
             
             try:
                 # Perform download in thread to avoid blocking event loop
-                download_success = await asyncio.to_thread(
+                loop = asyncio.get_running_loop()
+                download_success = await loop.run_in_executor(
+                    self.executor,
                     self.loader.download,
                     self.directory_to_search,
                     serie_folder,
@@ -481,7 +494,9 @@ class SeriesApp:
         try:
             # Get total items to scan
             logger.info("Getting total items to scan...")
-            total_to_scan = await asyncio.to_thread(
+            loop = asyncio.get_running_loop()
+            total_to_scan = await loop.run_in_executor(
+                self.executor,
                 self.serie_scanner.get_total_to_scan
             )
             logger.info("Total folders to scan: %d", total_to_scan)
@@ -503,7 +518,10 @@ class SeriesApp:
             )
 
             # Reinitialize scanner
-            await asyncio.to_thread(self.serie_scanner.reinit)
+            await loop.run_in_executor(
+                self.executor,
+                self.serie_scanner.reinit
+            )
 
             def scan_progress_handler(progress_data):
                 """Handle scan progress events from scanner."""
@@ -528,7 +546,10 @@ class SeriesApp:
             
             try:
                 # Perform scan (file-based, returns results in scanner.keyDict)
-                await asyncio.to_thread(self.serie_scanner.scan)
+                await loop.run_in_executor(
+                    self.executor,
+                    self.serie_scanner.scan
+                )
             finally:
                 # Always unsubscribe after scan completes or fails
                 self.serie_scanner.unsubscribe_on_progress(
@@ -685,3 +706,14 @@ class SeriesApp:
         )
 
         return all_series
+
+    def shutdown(self) -> None:
+        """
+        Shutdown the thread pool executor.
+        
+        Should be called when the SeriesApp instance is no longer needed
+        to properly clean up resources.
+        """
+        if hasattr(self, 'executor'):
+            self.executor.shutdown(wait=True)
+            logger.info("ThreadPoolExecutor shut down successfully")