feat: improve remove background with edge smoothing, color decontamination, output formats

- Expose birefnet-hr-matting in UI (People/Ultra) and fix model defaults
  (People/Max now uses birefnet-matting for true alpha matting)
- Add output format selector (PNG/WebP/AVIF) with lossless alpha support
- Add edge smoothing post-processing (Off/Light/Medium/Strong) via
  morphological mask refinement to reduce gray halo artifacts
- Add color decontamination to remove background color spill from
  semi-transparent edge pixels
- Thread new settings through full stack: frontend -> API schema ->
  Python sidecar -> Sharp effects pipeline
- Add i18n keys for all 21 locales
- Add unit tests for new option serialization (3 tests)
- Add integration tests for new settings validation (4 tests)

packages/ai/src/background-removal.ts

@@ -8,6 +8,8 @@ import { type ProgressCallback, parseStdoutJson, runPythonWithProgress } from ".
 export interface RemoveBackgroundOptions {
   model?: string;
   backgroundColor?: string;
+  edgeRefine?: number;
+  decontaminate?: boolean;
 }
 
 const MAX_REMBG_PX = Number(process.env.MAX_REMBG_PX) || 2048;