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)

tests/integration/remove-background.test.ts

@@ -387,6 +387,102 @@ describe("Remove Background", () => {
     }
   });
 
+  it("accepts edge refinement and decontamination settings", async () => {
+    const { body, contentType } = createMultipartPayload([
+      { name: "file", filename: "test.png", contentType: "image/png", content: PNG },
+      {
+        name: "settings",
+        content: JSON.stringify({ edgeRefine: 2, decontaminate: true }),
+      },
+    ]);
+
+    const res = await app.inject({
+      method: "POST",
+      url: "/api/v1/tools/remove-background",
+      headers: {
+        authorization: `Bearer ${adminToken}`,
+        "content-type": contentType,
+      },
+      body,
+    });
+
+    expect([202, 501]).toContain(res.statusCode);
+  }, 60_000);
+
+  it("accepts output format settings", async () => {
+    const { body, contentType } = createMultipartPayload([
+      { name: "file", filename: "test.png", contentType: "image/png", content: PNG },
+      {
+        name: "settings",
+        content: JSON.stringify({ outputFormat: "webp" }),
+      },
+    ]);
+
+    const res = await app.inject({
+      method: "POST",
+      url: "/api/v1/tools/remove-background",
+      headers: {
+        authorization: `Bearer ${adminToken}`,
+        "content-type": contentType,
+      },
+      body,
+    });
+
+    expect([202, 501]).toContain(res.statusCode);
+  }, 60_000);
+
+  it("rejects edgeRefine out of range", async () => {
+    const { body, contentType } = createMultipartPayload([
+      { name: "file", filename: "test.png", contentType: "image/png", content: PNG },
+      {
+        name: "settings",
+        content: JSON.stringify({ edgeRefine: 5 }),
+      },
+    ]);
+
+    const res = await app.inject({
+      method: "POST",
+      url: "/api/v1/tools/remove-background",
+      headers: {
+        authorization: `Bearer ${adminToken}`,
+        "content-type": contentType,
+      },
+      body,
+    });
+
+    expect([400, 501]).toContain(res.statusCode);
+    if (res.statusCode === 400) {
+      const result = JSON.parse(res.body);
+      expect(result.error).toMatch(/invalid settings/i);
+    }
+  });
+
+  it("rejects invalid output format", async () => {
+    const { body, contentType } = createMultipartPayload([
+      { name: "file", filename: "test.png", contentType: "image/png", content: PNG },
+      {
+        name: "settings",
+        content: JSON.stringify({ outputFormat: "gif" }),
+      },
+    ]);
+
+    const res = await app.inject({
+      method: "POST",
+      url: "/api/v1/tools/remove-background",
+      headers: {
+        authorization: `Bearer ${adminToken}`,
+        "content-type": contentType,
+      },
+      body,
+    });
+
+    expect([400, 501]).toContain(res.statusCode);
+    if (res.statusCode === 400) {
+      const result = JSON.parse(res.body);
+      expect(result.error).toMatch(/invalid settings/i);
+    }
+  });
+
   it("rejects unauthenticated requests", async () => {
     const { body, contentType } = createMultipartPayload([
       { name: "file", filename: "test.png", contentType: "image/png", content: PNG },

tests/unit/ai/background-removal.test.ts

@@ -97,6 +97,35 @@ describe("removeBackground", () => {
       });
     });
 
+    it("serializes edgeRefine option into the args JSON", async () => {
+      await removeBackground(FAKE_INPUT, FAKE_OUTPUT_DIR, { edgeRefine: 2 });
+
+      const args = vi.mocked(runPythonWithProgress).mock.calls[0][1];
+      expect(JSON.parse(args[2])).toEqual({ edgeRefine: 2 });
+    });
+
+    it("serializes decontaminate option into the args JSON", async () => {
+      await removeBackground(FAKE_INPUT, FAKE_OUTPUT_DIR, { decontaminate: true });
+
+      const args = vi.mocked(runPythonWithProgress).mock.calls[0][1];
+      expect(JSON.parse(args[2])).toEqual({ decontaminate: true });
+    });
+
+    it("serializes all post-processing options together", async () => {
+      await removeBackground(FAKE_INPUT, FAKE_OUTPUT_DIR, {
+        model: "birefnet-matting",
+        edgeRefine: 1,
+        decontaminate: true,
+      });
+
+      const args = vi.mocked(runPythonWithProgress).mock.calls[0][1];
+      expect(JSON.parse(args[2])).toEqual({
+        model: "birefnet-matting",
+        edgeRefine: 1,
+        decontaminate: true,
+      });
+    });
+
     it("converts input to PNG via sharp before writing to disk", async () => {
       await removeBackground(FAKE_INPUT, FAKE_OUTPUT_DIR);