test: close the coverage gaps the first round left

Second pass over the suite, driven by the inventory in the gaps document. Nine
agents wrote suites in parallel against private databases, then a tenth read all
of it adversarially and five of its findings were fixed.

  unit + component  2076 -> 2079 (+888 over the round)
  api                647 -> 1015
  e2e                 18 -> 29

What was closed:

- lib/route-access.ts, the page-level authorization layer, went from zero tests
  to 48. Every API route was guarded and none of the pages were.
- The five media proxy routes now have a real 2xx beside every 403. The blocker
  was the positive control, solved by stubbing r2Client.send() and leaving
  lib/r2-media-proxy.ts itself real.
- Every remaining server-side lib module: invitations, email verification, the
  upload tokens, the logger, request origin, the whole R2 and Bunny lifecycle,
  notifications and admin stats.
- Six video-page hooks, and the chunking arithmetic extracted out of
  lib/client/r2-video-upload.ts as a pure module.
- Five end-to-end flows: workspace members, bulk operations, the admin area,
  player interaction and failure recovery.

Three things about the harness itself turned out to be wrong:

- Two @/lib/r2 stubs in tests/setup/api.ts had the wrong return shape, so every
  route reaching finalizeR2VideoUpload silently took the "not a valid video"
  branch and no test noticed.
- The auth matrix asserted only "not 2xx", which two entries satisfied without
  their guard existing. It now requires 401 or 403, which makes both
  load-bearing, and all 60 routes pass the stricter form.
- Both admin API routes had no positive control anywhere: replacing their guard
  with an unconditional refusal left the entire suite green. Found by the
  adversarial review, now covered.

Process:

- bun run test:mutation runs StrykerJS over the authorization and validation
  modules. Diagnostic, not a gate, weekly in CI rather than on a push.
- playwright.config.ts gains an opt-in webkit project for the player spec.
- AGENTS.md now requires a batch of new tests to be reviewed by somebody who
  did not write them.

Only two production files change, both deliberate: lib/auth.ts loses a verbatim
copy of its own permission formulas, and lib/client/r2-video-upload.ts calls the
extracted arithmetic. No behaviour change in either.

vitest.mutation.config.ts

@@ -0,0 +1,32 @@
+import { defineConfig } from 'vitest/config';
+import tsconfigPaths from 'vite-tsconfig-paths';
+
+// The Vitest config StrykerJS runs against, kept apart from vitest.config.ts on
+// purpose.
+//
+// Only the `unit` project belongs here. Stryker restarts the suite once per
+// surviving mutant across several concurrent workers, and the `api` project
+// cannot take that: it shares one Postgres, empties every table between tests,
+// and therefore runs with `fileParallelism: false`. Pointing Stryker at it would
+// either serialise the whole run into something nobody waits for, or let two
+// workers truncate each other's rows and report the resulting failures as killed
+// mutants, which is a false pass.
+//
+// So mutation coverage is scoped to modules the unit project genuinely covers.
+// `stryker.config.json` lists them explicitly rather than globbing `lib/`, for
+// the same reason: a file whose only coverage is an API integration test would
+// report every one of its mutants as survived, and a report that is mostly noise
+// gets ignored.
+export default defineConfig({
+  plugins: [tsconfigPaths()],
+  test: {
+    environment: 'node',
+    include: ['tests/unit/**/*.test.ts'],
+    // Same reason as the root config: next-auth's lib/env.js imports the
+    // extensionless specifier 'next/server', which Node's ESM resolver cannot
+    // resolve, so it has to go through Vite's resolver instead of being
+    // externalised. Stryker runs under node, never under bun, so this is not
+    // optional here the way it appears to be locally.
+    server: { deps: { inline: [/next-auth/] } },
+  },
+});