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/setup/per-fork-env.ts

@@ -0,0 +1,13 @@
+import crypto from "node:crypto";
+import os from "node:os";
+import path from "node:path";
+
+// Each Vitest fork gets its own SQLite DB + workspace so test files can run
+// in parallel. setupFiles run before any test file (and therefore before any
+// app module) loads, so apps/api/src/config.ts captures the per-fork paths.
+const forkDir = path.join(
+  os.tmpdir(),
+  `SnapOtter-test-${process.pid}-${crypto.randomUUID().slice(0, 8)}`,
+);
+process.env.DB_PATH = path.join(forkDir, "test.db");
+process.env.WORKSPACE_PATH = path.join(forkDir, "workspace");