change thumbnail format from PNG to JPEG to reduce size

regenerate missing thumbnails on startup
one time force regeneration of all thumbnails

komga/src/main/kotlin/org/gotson/komga/domain/service/BookParser.kt

@@ -36,7 +36,7 @@ class BookParser(
   private val natSortComparator: Comparator<String> = CaseInsensitiveSimpleNaturalComparator.getInstance()
 
   private val thumbnailSize = 300
-  private val thumbnailFormat = "png"
+  private val thumbnailFormat = "jpeg"
 
   @Throws(UnsupportedMediaTypeException::class)
   fun parse(book: Book): BookMetadata {

komga/src/main/kotlin/org/gotson/komga/interfaces/scheduler/PeriodicScannerController.kt

@@ -26,4 +26,13 @@ class PeriodicScannerController(
       logger.warn { "Another scan is already running, skipping" }
     }
   }
-}
+
+  @EventListener(ApplicationReadyEvent::class)
+  fun regenerateMissingThumbnails() {
+    try {
+      asyncOrchestrator.regenerateMissingThumbnails()
+    } catch (e: RejectedExecutionException) {
+      logger.warn { "Thumbnail regeneration task is already running" }
+    }
+  }
+}

komga/src/main/kotlin/org/gotson/komga/interfaces/web/rest/SeriesController.kt

@@ -104,7 +104,7 @@ class SeriesController(
   ): SeriesDto =
       seriesRepository.findByIdOrNull(id)?.toDto() ?: throw ResponseStatusException(HttpStatus.NOT_FOUND)
 
-  @GetMapping(value = ["{seriesId}/thumbnail"], produces = [MediaType.IMAGE_PNG_VALUE])
+  @GetMapping(value = ["{seriesId}/thumbnail"], produces = [MediaType.IMAGE_JPEG_VALUE])
   fun getSeriesThumbnail(
       @PathVariable seriesId: Long
   ): ByteArray {

komga/src/main/resources/db/migration/V20191010153833__delete_thumbnails_to_force_regeneration_in_jpeg.sql

@@ -0,0 +1,2 @@
+update book_metadata
+set thumbnail = null;