feat: improve demo upload flow and model selection

新增 demo 頁面的模型切換功能，支援在同一張圖片上切換不同模型重新推論，
並調整預覽圖片的縮放與顯示尺寸，避免被外框裁切。

改寫 multipart 表單解析流程，支援重用既有預覽圖並放寬上傳大小限制，
同時修正手動解析表單時無法正確辨識上傳檔案的問題。

app.py

@@ -8,6 +8,7 @@ from fastapi import FastAPI, HTTPException, Request, UploadFile
 from fastapi.responses import HTMLResponse
 from fastapi.templating import Jinja2Templates
 from PIL import Image, UnidentifiedImageError
+from starlette.datastructures import UploadFile as StarletteUploadFile
 
 from model_service import MODEL_CONFIGS, get_model_config, get_model_service
 
@@ -116,6 +117,12 @@ async def _predict_upload(
     return result, data
 
 
+def _coerce_upload_file(value: object) -> UploadFile | None:
+    if isinstance(value, (UploadFile, StarletteUploadFile)):
+        return value
+    return None
+
+
 async def _parse_demo_form(request: Request) -> tuple[UploadFile | None, str, str | None]:
     try:
         form = await request.form(max_part_size=MAX_UPLOAD_SIZE)
@@ -124,8 +131,7 @@ async def _parse_demo_form(request: Request) -> tuple[UploadFile | None, str, st
     except Exception as exc:
         raise HTTPException(status_code=400, detail=str(exc)) from exc
 
-    file_value = form.get("file")
-    file = file_value if isinstance(file_value, UploadFile) else None
+    file = _coerce_upload_file(form.get("file"))
     model_name = str(form.get("model_name") or ACTIVE_MODEL_CONFIG.name)
     existing_image_data_url = form.get("existing_image_data_url")
     if existing_image_data_url is not None:
@@ -141,8 +147,7 @@ async def _parse_predict_form(request: Request) -> tuple[UploadFile | None, str]
     except Exception as exc:
         raise HTTPException(status_code=400, detail=str(exc)) from exc
 
-    file_value = form.get("file")
-    file = file_value if isinstance(file_value, UploadFile) else None
+    file = _coerce_upload_file(form.get("file"))
     model_name = str(form.get("model_name") or ACTIVE_MODEL_CONFIG.name)
     return file, model_name