feat: add /image endpoint for chapter pages with Content-Length header

New GET endpoint at /api/v1/manga/{mangaId}/chapter/{chapterIndex}/page/{index}/image
that buffers the image into a byte array and sets Content-Length, enabling
progress tracking in Tachiyomi-based Android extensions.

server/src/main/kotlin/suwayomi/tachidesk/manga/MangaAPI.kt

@@ -79,6 +79,7 @@ object MangaAPI {
             patch("{mangaId}/chapter/{chapterIndex}/meta", MangaController.chapterMeta)
 
             get("{mangaId}/chapter/{chapterIndex}/page/{index}", MangaController.pageRetrieve)
+            get("{mangaId}/chapter/{chapterIndex}/page/{index}/image", MangaController.pageImage)
         }
 
         path("chapter") {

server/src/main/kotlin/suwayomi/tachidesk/manga/controller/MangaController.kt

@@ -503,6 +503,46 @@ object MangaController {
             },
         )
 
+    /** Get page image binary with Content-Length for progress tracking */
+    val pageImage =
+        handler(
+            pathParam<Int>("mangaId"),
+            pathParam<Int>("chapterIndex"),
+            pathParam<Int>("index"),
+            queryParam<String?>("format"),
+            documentWith = {
+                withOperation {
+                    summary("Get a chapter page image")
+                    description("Returns the raw image binary with Content-Length header for progress tracking.")
+                }
+            },
+            behaviorOf = { ctx, mangaId, chapterIndex, index, format ->
+                ctx.getAttribute(Attribute.TachideskUser).requireUser()
+
+                ctx.future {
+                    future {
+                        Page.getPageImageServe(
+                            mangaId = mangaId,
+                            chapterIndex = chapterIndex,
+                            index = index,
+                            format = format,
+                        )
+                    }.thenApply { (stream, mimeType) ->
+                        val bytes = stream.readBytes()
+                        ctx.header("content-type", mimeType)
+                        ctx.header("content-length", bytes.size.toString())
+                        val httpCacheSeconds = 1.days.inWholeSeconds
+                        ctx.header("cache-control", "max-age=$httpCacheSeconds")
+                        ctx.result(bytes)
+                    }
+                }
+            },
+            withResults = {
+                image(HttpStatus.OK)
+                httpCode(HttpStatus.NOT_FOUND)
+            },
+        )
+
     val downloadChapter =
         handler(
             pathParam<Int>("chapterId"),