test: comprehensive test coverage expansion (+965 tests)

Add 42 new test files covering all untested tool routes, image engine
internals, AI sidecar bridge, Zustand stores, and cross-format
compatibility. Expand e2e-docker suite with 7 spec files covering all
48 tools against a real Docker container.

Unit tests:
- Image engine: format detection, MIME mapping, metadata parsing, pipeline
- AI bridge: sidecar lifecycle, all 11 tool functions (mocked)
- Web stores: 14 Zustand stores (collage, settings, features, analytics, etc.)
- API helpers: format decoders, page range, file validation

Integration tests:
- 25 tool routes that had zero dedicated tests
- Cross-format matrix: 17 input formats x 3 tools
- Edge cases: zero-byte files, corrupted headers, path traversal, XSS, SQL injection
- Concurrent request handling and pipeline edge cases

E2E-Docker (Playwright against real container):
- 7 spec files: essential, adjustment, conversion, creative, utility, AI, pipeline
- Custom buildMultipart helper for multi-file tool uploads
- AI tools gracefully skip when sidecar not installed

Fixtures:
- Organized test media: formats/ (18 formats) + content/ (17 content types)
- Reduced from 3.1 GB unorganized samples to 33 MB structured fixtures

Bug fix:
- color-adjustments: gamma exposure used invalid single-param gamma() for
  positive values; fixed to use two-param gamma(gammaIn, gammaOut) form

tests/unit/api/format-decoders.test.ts

@@ -0,0 +1,94 @@
+import { describe, expect, it } from "vitest";
+import { decodeToSharpCompat, needsCliDecode } from "../../../apps/api/src/lib/format-decoders.js";
+
+// ==========================================================================
+// needsCliDecode
+// ==========================================================================
+
+describe("needsCliDecode", () => {
+  it("returns true for raw format", () => {
+    expect(needsCliDecode("raw")).toBe(true);
+  });
+
+  it("returns true for ico format", () => {
+    expect(needsCliDecode("ico")).toBe(true);
+  });
+
+  it("returns true for tga format", () => {
+    expect(needsCliDecode("tga")).toBe(true);
+  });
+
+  it("returns true for psd format", () => {
+    expect(needsCliDecode("psd")).toBe(true);
+  });
+
+  it("returns true for exr format", () => {
+    expect(needsCliDecode("exr")).toBe(true);
+  });
+
+  it("returns true for hdr format", () => {
+    expect(needsCliDecode("hdr")).toBe(true);
+  });
+
+  it("returns false for jpeg format", () => {
+    expect(needsCliDecode("jpeg")).toBe(false);
+  });
+
+  it("returns false for png format", () => {
+    expect(needsCliDecode("png")).toBe(false);
+  });
+
+  it("returns false for webp format", () => {
+    expect(needsCliDecode("webp")).toBe(false);
+  });
+
+  it("returns false for gif format", () => {
+    expect(needsCliDecode("gif")).toBe(false);
+  });
+
+  it("returns false for avif format", () => {
+    expect(needsCliDecode("avif")).toBe(false);
+  });
+
+  it("returns false for svg format", () => {
+    expect(needsCliDecode("svg")).toBe(false);
+  });
+
+  it("returns false for empty string", () => {
+    expect(needsCliDecode("")).toBe(false);
+  });
+
+  it("returns false for unknown format", () => {
+    expect(needsCliDecode("bmp")).toBe(false);
+  });
+});
+
+// ==========================================================================
+// decodeToSharpCompat — routing logic (default passthrough)
+// ==========================================================================
+
+describe("decodeToSharpCompat", () => {
+  it("returns buffer unchanged for unknown/native formats", async () => {
+    const buf = Buffer.from("test data");
+    const result = await decodeToSharpCompat(buf, "jpeg");
+    expect(result).toBe(buf);
+  });
+
+  it("returns buffer unchanged for png format", async () => {
+    const buf = Buffer.from("png data");
+    const result = await decodeToSharpCompat(buf, "png");
+    expect(result).toBe(buf);
+  });
+
+  it("returns buffer unchanged for empty format string", async () => {
+    const buf = Buffer.from("some bytes");
+    const result = await decodeToSharpCompat(buf, "");
+    expect(result).toBe(buf);
+  });
+
+  it("returns buffer unchanged for webp format", async () => {
+    const buf = Buffer.from("webp");
+    const result = await decodeToSharpCompat(buf, "webp");
+    expect(result).toBe(buf);
+  });
+});