Add an ephemeral similarity check page

- /similar (nav: Similar): drop a file to get the exact MD5 match, the
  perceptual matches against the library, and e621 IQDB candidates
  (auto-run for images when credentials are configured). Read-only —
  nothing enters the library.
- SimilarityCheck model + /api/similarity/ (create/list/retrieve/delete)
  with signed preview URLs and an expires_at timestamp.
- Temp files are wiped on startup (AppConfig.ready, file-only so no
  database access during initialization), lazily past
  SIMILARITY_TTL_MINUTES (default 30, env-overridable), on delete, and
  by manage.py cleanup_similarity.
- uploadFile() takes a target path; .env.example documents the TTL.

frontend/src/lib/api.ts

@@ -85,11 +85,12 @@ export async function api<T>(path: string, options: ApiOptions = {}): Promise<T>
   return (await response.json()) as T;
 }
 
-export function uploadFile(
+export function uploadFile<T = TempUpload>(
   file: File,
   fields: Record<string, string> = {},
   onProgress?: (percent: number) => void,
-): Promise<TempUpload> {
+  path = "/api/uploads/",
+): Promise<T> {
   return new Promise((resolve, reject) => {
     const form = new FormData();
     form.append("file", file);
@@ -98,7 +99,7 @@ export function uploadFile(
     }
 
     const request = new XMLHttpRequest();
-    request.open("POST", "/api/uploads/");
+    request.open("POST", path);
     const token = getToken();
     if (token) request.setRequestHeader("Authorization", `Token ${token}`);
     request.upload.addEventListener("progress", (event) => {
@@ -114,7 +115,7 @@ export function uploadFile(
         data = null;
       }
       if (request.status >= 200 && request.status < 300) {
-        resolve(data as TempUpload);
+        resolve(data as T);
       } else {
         reject(new ApiError(request.status, data));
       }