fix(compress-pdf): land close to the target size, honestly (#522)

Target-size compression had only a coarse DPI lever, so it undershot badly (a 350KB target could land at 216KB) and silently missed unreachable targets. Adds JPEG quality as a second lever (forced re-encode so it bites on JPEG scans), folds both into one monotonic quality axis that target-size binary-searches, reports targetMet honestly in the panel across 21 locales, and flips the tool to async for the extra passes. Quality-mode output sizes shift intentionally (slider now drives JPEG quality at full resolution in its top half).

tests/unit/api/compress-pdf-params.test.ts

@@ -0,0 +1,30 @@
+import { describe, expect, it } from "vitest";
+import { paramsForQuality } from "../../../apps/api/src/routes/tools/compress-pdf.js";
+
+describe("paramsForQuality", () => {
+  it("endpoints: q=100 preserves resolution at best quality; q=1 is smallest", () => {
+    const hi = paramsForQuality(100);
+    expect(hi.dpi).toBe(300);
+    expect(hi.qFactor).toBeCloseTo(0.1, 2);
+    const lo = paramsForQuality(1);
+    expect(lo.dpi).toBeLessThanOrEqual(30);
+    expect(lo.qFactor).toBeGreaterThan(2.0);
+  });
+
+  it("is monotonic in size: dpi non-decreasing and qFactor non-increasing as q rises", () => {
+    let prevDpi = 0;
+    let prevQf = Number.POSITIVE_INFINITY;
+    for (let q = 1; q <= 100; q++) {
+      const { dpi, qFactor } = paramsForQuality(q);
+      expect(dpi).toBeGreaterThanOrEqual(prevDpi);
+      expect(qFactor).toBeLessThanOrEqual(prevQf + 1e-9);
+      prevDpi = dpi;
+      prevQf = qFactor;
+    }
+  });
+
+  it("clamps out-of-range input", () => {
+    expect(paramsForQuality(0)).toEqual(paramsForQuality(1));
+    expect(paramsForQuality(200)).toEqual(paramsForQuality(100));
+  });
+});