test: testing overhaul -- CI e2e gates, parallel suites, generated matrices, mutation testing (#215)

Closes the "e2e never runs in CI" hole. Adds per-PR e2e smoke gate,
nightly full-suite workflows, parallel vitest forks (per-fork DBs),
Playwright parallel/serial/visual projects against production builds,
metadata-generated test suites (drift guards, hostile inputs, format
matrix, pairwise settings, property-based fuzz), Stryker mutation
testing, Schemathesis API fuzz, coverage ratchet, and fixes for three
session-poisoning bugs that caused 200+ serial-bucket failures.

Bug fix included: favicon/split/bulk-rename could hang clients forever
when ZIP streaming failed after reply.hijack().

tests/helpers/tool-default-settings.ts

@@ -0,0 +1,22 @@
+/**
+ * Minimal valid settings per tool, used by the generated matrices
+ * (format-matrix-generated, hostile-inputs) when posting to tool routes.
+ *
+ * Default is {} (most schemas make every field optional). Tools whose schema
+ * rejects {} get an explicit minimal override here. The "defaults are valid"
+ * test in format-matrix-generated.test.ts safeParses every entry against the
+ * live schema, so a schema change that invalidates an entry fails at PR time
+ * and names the tool.
+ */
+export const TOOL_SETTINGS_OVERRIDES: Record<string, unknown> = {
+  resize: { width: 64 },
+  crop: { left: 0, top: 0, width: 50, height: 50 },
+  convert: { format: "png" },
+  "watermark-text": { text: "Test" },
+  "text-overlay": { text: "Test" },
+  "passport-photo": { countryCode: "us" },
+};
+
+export function defaultSettingsFor(toolId: string): unknown {
+  return TOOL_SETTINGS_OVERRIDES[toolId] ?? {};
+}