feat(tools): remove background from animated GIFs (WebP, APNG) (#502)

Adds a dedicated remove-gif-background AI tool: removes the background from an animated GIF, WebP, or APNG frame by frame and reassembles a transparent (or composited) animation in WebP, APNG, or GIF, with full per-frame effects. Reuses the background-removal bundle. Verified end-to-end with the real rembg model.

Closes #496.

tests/unit/api/animation-detect.test.ts

@@ -0,0 +1,29 @@
+import { describe, expect, it } from "vitest";
+import { detectAnimation } from "../../../apps/api/src/lib/animation-detect.js";
+import { fixtures, readFixture } from "../../fixtures/index.js";
+
+describe("detectAnimation", () => {
+  it("counts GIF frames via Sharp", async () => {
+    const r = await detectAnimation(readFixture(fixtures.image.animated.gif), "animated.gif");
+    expect(r.animated).toBe(true);
+    expect(r.frames).toBeGreaterThan(1);
+  });
+
+  it("counts animated WebP frames via Sharp", async () => {
+    const r = await detectAnimation(readFixture(fixtures.image.animated.webp), "animated.webp");
+    expect(r.animated).toBe(true);
+    expect(r.frames).toBeGreaterThan(1);
+  });
+
+  it("counts APNG frames via the acTL chunk", async () => {
+    const r = await detectAnimation(readFixture(fixtures.image.animated.apng), "animated.apng");
+    expect(r.animated).toBe(true);
+    expect(r.frames).toBe(4);
+  });
+
+  it("treats a still PNG as not animated", async () => {
+    const r = await detectAnimation(readFixture(fixtures.image.base.png200), "still.png");
+    expect(r.animated).toBe(false);
+    expect(r.frames).toBe(1);
+  });
+});