feat(v5.2.0): finalize Infrastructure & Polish (tests isolation, xdist, lint, CI)

tests/conftest.py: mutate the settings singleton (instead of rebinding) so DB + backup dir are isolated per test -> pytest-xdist safe.
Real backup tests (snapshot/prune/admin API) and OAuth mock tests (Gitea/GitHub/link) replace the previous skips.
init_db() now also creates webhook_subscriptions (full schema without the FastAPI lifespan).
ruff check is clean; .eslintrc.json migrated to eslint.config.mjs (flat config).
CI: lint job (ruff + eslint), parallel tests (-n auto), run on every branch push.
VERSION 5.11.1.

app/main.py

@@ -85,7 +85,7 @@ async def lifespan(_app: FastAPI):
     from app.services.trash import trash_purge_scheduler
     trash_task = asyncio.create_task(trash_purge_scheduler())
 
-    logger.info("FlowDeck v5.10.0 started on port %d", settings.app_port)
+    logger.info("FlowDeck v5.11.1 started on port %d", settings.app_port)
     try:
         yield
     finally:
@@ -100,7 +100,7 @@ async def lifespan(_app: FastAPI):
 
 app = FastAPI(
     title="FlowDeck",
-    version="5.10.0",
+    version="5.11.1",
     docs_url="/docs" if settings.log_level == "DEBUG" else None,
     redoc_url=None,
     lifespan=lifespan,