build: relocate the generated clients and the system test suite

[ExtraToast]

Why

services/ held two things that do not ship. services/api/clients/brevo and
services/api/clients/discord carry no committed source at all — one
build.gradle.kts each, generating a Java client from libs/openapi-specs at
build time. They are third-party surfaces, not part of the api service.
services/system-tests is 62 Kotlin files of Playwright and Cucumber driving
the assembled stack over HTTP — a test suite, not a deployable.

settings.gradle.kts also still carried a note claiming libs/kotlin-common
was an empty skeleton and asking the reader to add a dependency that
services/api/build.gradle.kts has declared since the helpers landed.

What this achieves

services/ now holds only what ships: api, frontend, stalwart, vault.
The generated clients sit next to the specs they are generated from, so a spec
change and the client it regenerates are one directory apart. The system test
suite has a home that names what it is, leaving room for further test projects
under tests/ without them reading as services.

libs/kotlin-common stays where it is — its dependencies are declared
compileOnly against the Spring BOM, which keeps it deliberately
service-agnostic; only the stale note about it is gone.

libs/openapi-specs also stays as it is: a plain directory read by
build-logic, scripts/openapi-common.sh and the spec-sync workflow, not a
Gradle module.

How

Moved, as git renames with no content change to the sources:

• services/api/clients/brevo -> libs/clients/brevo
• services/api/clients/discord -> libs/clients/discord
• services/system-tests -> tests/system (73 files)

Gradle project paths follow the directories: :services:api:clients:brevo and
:services:api:clients:discord become :libs:clients:brevo and
:libs:clients:discord; :services:system-tests becomes :tests:system.

Path updates:

• settings.gradle.kts — new includes, and the stale libs:kotlin-common note deleted.
• services/api/build.gradle.kts — both project(":libs:clients:…") dependencies.
• libs/clients/discord/build.gradle.kts — the root-relative specPath pointing at its own filtered spec.
• scripts/openapi-common.sh — :libs:clients:brevo:generate.
• .github/workflows/validate.yml — the tests/** push filter, the warm-cache targets in the api-static job, the :tests:system:test command in the six-shard system-tests matrix (shard: [1, 2, 3, 4, 5, 6]), the :tests:system:acceptanceTest command in acceptance-features, and the report artifact paths for both.
• .github/diff-stats.yml — the services/api/clients/** rule replaced by libs/clients/** in the libs block, and the system-test glob repointed at tests/system/**. The system-tests service label is unchanged, so .github/scripts/pr_diff_stats.py needs no edit.
• build-logic/src/main/kotlin/testing-conventions.gradle.kts — the comment naming the extracted project.
• .gitignore — the env-file guard was scoped to services/ only, so the rules follow the suite: tests/**/.*.env and tests/**/*.env sit alongside the existing pair.
• libs/openapi-specs/README.md, tests/system/src/test/resources/features/README.md, platform/docs/bringup-v2.md, docs/adr/api/ADR-011, docs/adr/testing/ADR-001 and ADR-006, and the five docs/flows/*/README.md files that link into the feature files.

.github/scripts/start-system-test-stack.sh, docker-compose.ci.yml and
.github/actions/setup-java-gradle/action.yml name the CI job rather than a
path, and the job is still called system-tests, so they are untouched.

Closes #719

---

Diff breakdown — █ added ░ removed, scaled to the largest row.

api                                                 +2     -2    1
  build & config     █░                             +2     -2    1

libs                                                +1     -1    2
  production         █░                             +1     -1    2
  generated          █░                             +2     -2    1  ~

platform                                            +2     -2    1
  infrastructure     █░                             +2     -2    1

ci                                                 +19    -23    5
  build & config     ██████████░░░░░░░░░░░░        +19    -23    5

repo                                                +3     -1    1
  build & config     █░                             +3     -1    1

docs                                               +25    -25    9
  docs               █████████████░░░░░░░░░░░░░    +25    -25    9

other                                               +1     -1   72
  unclassified       █░                             +1     -1   72

──────────────────────────────────────────────────────────────────
production                                          +1     -1
tests                                               +0     -0  0.00 test lines per prod line
total (hand-written)                               +53    -55  91 files
~ generated (excluded)                              +2     -2  1 file