docs(infra): conclude runner + deploy investigation — both outside the repo

Moves the report to docs/infrastructure/runner-investigation.md with the
requested structure (evidence, experiments, hypothesis, confidence, required
infrastructure changes, why application code is no longer suspected).

Decisive new experiment: the suite was run from a clean `git archive HEAD`
tree — byte-identical to CI's checkout, without the gitignored runtime dirs
(jobtracker.db, keys/, CvArtifacts/, backups/) that earlier local runs had
silently included. 10/10 pass in 1s. That removes the last difference between
the local tree and the runner, eliminating application code (~95% confidence).

Also establishes, by route probe, that production is healthy but stale:
/api/public-cv/{unknown} returns 404 locally (route exists, AllowAnonymous) but
401 on prod, same as a nonsense path — PublicCvController is absent, so Phase 4
and Phase 5 have never deployed. Production therefore never ran the faulty
migration: no half-built tables exist there and no data cleanup is needed.

Deploy is a second, separate infrastructure failure: the first attempt reached
deploy.sh (37s, consistent with the MariaDB crash since fixed), every attempt
since dies at 3s at SSH connection time while the host serves traffic normally.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/ci-deploy.yml

@@ -66,7 +66,7 @@ jobs:
       # log is not readable via the Gitea API). Those 10 tests pass on Windows, in a clean Linux
       # container, under a 1GB memory cap, with a custom-dir SDK and no DOTNET_ROOT, serially, and
       # under a hostile locale/timezone -- so the trigger is specific to this runner host.
-      # See docs/ci-runner-investigation.md. Collection parallelism stays off for determinism, the
+      # See docs/infrastructure/runner-investigation.md. Collection parallelism stays off for determinism, the
       # same reason the frontend runs --runInBand. Every test still runs; nothing is filtered.
       - name: Test backend
         run: dotnet test JobTrackerApi.Tests/JobTrackerApi.Tests.csproj --configuration Release --no-build -- xUnit.parallelizeTestCollections=false xUnit.maxParallelThreads=1