ci(mutation): make Stryker report upload best-effort (refs #62)

The Gitea artifact backend returns 500 to actions/upload-artifact@v3 (server-side,
distinct from the @v4 GHES guard). With if: always() that 500 failed the whole
mutation job even though the ratchet passed — red on main and on every PR. Mark the
three report uploads continue-on-error: true so the mutation *gate* stays the Stryker
ratchet (make mutation's exit code), not the report upload. Documented in
gitea-actions-gotchas.md §4.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yaml

@@ -51,25 +51,30 @@ jobs:
         with:
           dotnet-version: '10.0.x'
       - run: make mutation
-      # Publish the Stryker HTML report. `if: always()` uploads it even when the
+      # Publish the Stryker HTML reports. `if: always()` uploads them even when the
       # ratchet fails — that is exactly when you want to inspect the survivors.
-      # Glob handles Stryker's non-deterministic StrykerOutput/<timestamp>/ dir.
+      # `continue-on-error` keeps the upload best-effort: the mutation *gate* is the
-      # Pinned to @v3 deliberately: @v4 refuses to run on Gitea (GHES guard) —
+      # ratchet (make mutation's exit code), not the report, so a Gitea artifact-backend
-      # see docs/runbooks/gitea-actions-gotchas.md §4.
+      # 500 must not fail the job (gitea-actions-gotchas.md §4). Glob handles Stryker's
+      # non-deterministic StrykerOutput/<timestamp>/ dir. Pinned @v3: @v4's bundled
+      # @actions/artifact hard-aborts on non-github.com (GHES guard) — see the runbook.
       - uses: https://github.com/actions/upload-artifact@v3
         if: always()
+        continue-on-error: true
         with:
           name: acl-mutation-report
           path: services/acl/StrykerOutput/**/reports/mutation-report.html
           if-no-files-found: warn
       - uses: https://github.com/actions/upload-artifact@v3
         if: always()
+        continue-on-error: true
         with:
           name: event-subscriber-mutation-report
           path: services/event-subscriber/StrykerOutput/**/reports/mutation-report.html
           if-no-files-found: warn
       - uses: https://github.com/actions/upload-artifact@v3
         if: always()
+        continue-on-error: true
         with:
           name: domain-mutation-report
           path: services/domain/StrykerOutput/**/reports/mutation-report.html

docs/runbooks/gitea-actions-gotchas.md

@@ -14,6 +14,7 @@ those containers share a filesystem — or a `localhost` — breaks.
 | `docker compose up --wait` is unsupported / flaky | poll health with `docker inspect` | `infra/wait-healthy.sh` |
 | `pg_isready` passes before PostGIS is ready | add a `PostGIS_Version()` probe | the db healthchecks |
 | `upload-artifact@v4` fails ("not supported on GHES") | pin `@v3` | `.gitea/workflows/ci.yaml` (`mutation` job) |
+| `upload-artifact@v3` fails with "Artifact service responded with 500" | mark the upload `continue-on-error: true` (server-side; issue #62) | `.gitea/workflows/ci.yaml` (`mutation` job) |
 
 ---
 
@@ -125,6 +126,22 @@ guard. Inputs are the same (`name`, `path`, `if-no-files-found`), so it is a dro
 swap. Do **not** bump to `@v4` until act_runner advertises github.com-compatible
 artifact support.
 
+**Second failure mode — the server's artifact backend returns 500.** Even on the
+correctly-pinned `@v3`, uploads can fail with:
+
+```
+Create Artifact Container - Attempt 5 of 5 failed with error: Artifact service responded with 500
+::error::Create Artifact Container failed: Artifact service responded with 500
+```
+
+This is the **Gitea server's** artifact storage failing (not the action's GHES guard),
+so it is outside the repo's control. Because the `mutation` job's upload steps run with
+`if: always()`, that 500 would fail the job even though the ratchet passed. **Fix:** mark
+the uploads `continue-on-error: true` (issue #62). The mutation *gate* is the Stryker
+ratchet — `make mutation`'s exit code fails the job on a real regression — so the report
+upload is best-effort: when the server's artifact storage is restored, reports publish
+again with no workflow change.
+
 ---
 
 ## 5. A runner process can't reach a service container's published port