fix: adopt sharp 0.35.2+ by centralizing the FormatEnum key type (#362)

Centralizes the sharp format key type into a single `SharpFormat` alias derived from `toFormat()`'s signature (exported from @snapotter/image-engine, imported by the API consumers), replacing the duplicated `keyof FormatEnum` definitions. Adds convert/compress format round-trip tests covering webp/avif/png/jpeg. Part of #325.

apps/api/src/lib/output-format.ts

@@ -1,6 +1,5 @@
-import sharp, { type FormatEnum } from "sharp";
-
-export type SharpFormat = Extract<keyof FormatEnum, string> | "avif";
+import type { SharpFormat } from "@snapotter/image-engine";
+import sharp from "sharp";
 
 export interface OutputFormat {
   format: SharpFormat;