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.

stryker.config.json

@@ -0,0 +1,61 @@
+{
+  "$schema": "./node_modules/@stryker-mutator/core/schema/stryker-schema.json",
+  "_comment": [
+    "Mutation testing. Stryker rewrites one line of a source file at a time and reruns",
+    "the suite; a mutant that survives is a line no test disagrees with, which is the",
+    "machine-checkable version of 'this test cannot fail'.",
+    "",
+    "This suite was built with three tests that could not fail, all found by hand. This",
+    "config is what finds the fourth. It is diagnostic and it does not gate a merge: see",
+    "`break: null` below, and TESTING.md section 11 for the same reasoning applied to",
+    "coverage.",
+    "",
+    "Run it with `bun run test:mutation`, which puts it in a node container because",
+    "Stryker needs node and the report lands in reports/mutation/. CI runs it weekly and",
+    "on demand, never on a push, because a full run is minutes rather than seconds."
+  ],
+  "packageManager": "npm",
+  "testRunner": "vitest",
+  "vitest": {
+    "configFile": "vitest.mutation.config.ts"
+  },
+  "reporters": ["html", "clear-text", "progress"],
+  "htmlReporter": {
+    "fileName": "reports/mutation/index.html"
+  },
+  "coverageAnalysis": "perTest",
+  "timeoutMS": 60000,
+  "concurrency": 4,
+  "_mutate_comment": [
+    "Listed one by one rather than globbed as `lib/**`, and the reason matters. Stryker",
+    "reports every mutant in a file with no unit coverage as survived, so globbing the",
+    "whole of lib/ would bury the real findings under modules whose only coverage is an",
+    "API integration test that this config deliberately does not run (see",
+    "vitest.mutation.config.ts). A report that is mostly noise gets ignored.",
+    "",
+    "So the list is the authorization and input-validation surface, the places where a",
+    "test that cannot fail is actually dangerous. Add a module here once it has real",
+    "unit coverage."
+  ],
+  "mutate": [
+    "lib/route-access.ts",
+    "lib/rate-limit.ts",
+    "lib/share-links.ts",
+    "lib/r2-upload-token.ts",
+    "lib/bunny-upload-token.ts",
+    "lib/request-origin.ts",
+    "lib/logger.ts",
+    "lib/validation.ts",
+    "lib/email-validation.ts",
+    "lib/image-upload-validation.ts",
+    "lib/video-upload-validation.ts",
+    "lib/guest-identity.ts",
+    "lib/billing.ts",
+    "lib/content-security-policy.ts"
+  ],
+  "thresholds": {
+    "high": 85,
+    "low": 70,
+    "break": null
+  }
+}