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.

tests/conftest.py

@@ -1,9 +1,15 @@
-"""FlowDeck — pytest fixtures and configuration."""
+"""FlowDeck — pytest fixtures and configuration.
+
+Each test gets a fresh, isolated SQLite database and backup directory so the
+suite is safe to run in parallel (``pytest -n auto``): workers never share a
+database file, and no state leaks between tests.
+"""
 import os
 import tempfile
 from pathlib import Path
 
 import pytest
+from fastapi.testclient import TestClient
 
 
 @pytest.fixture
@@ -15,7 +21,7 @@ def client():
 
     backup_dir = tempfile.mkdtemp(prefix="fd_backups_")
 
-    # Set env BEFORE importing app modules (config reads at import time)
+    # Set env BEFORE importing app modules (config reads at import time).
     os.environ["DATABASE_URL"] = f"sqlite:///{db_path}"
     os.environ["APP_SECRET_KEY"] = "test-secret-for-tests"
     os.environ["RATE_LIMIT_ENABLED"] = "false"
@@ -23,9 +29,21 @@ def client():
     os.environ["BACKUP_DIR"] = backup_dir
     os.environ["PROJECT_SYNC_ENABLED"] = "false"
 
-    # Force config reload by clearing the cached Settings instance
+    # IMPORTANT: mutate the existing Settings singleton in place — do NOT rebind
+    # `app.config.settings`. Modules such as `app.services.backup` and
+    # `app.routers.auth` hold a direct reference imported at load time, so
+    # rebinding would leave them pointing at the stale defaults (this was the
+    # cause of the previously-skipped flaky backup tests).
     import app.config
-    app.config.settings = app.config.Settings()
+    s = app.config.settings
+    s.database_url = f"sqlite:///{db_path}"
+    s.app_secret_key = "test-secret-for-tests"
+    s.rate_limit_enabled = False
+    s.backup_enabled = True
+    s.backup_dir = backup_dir
+    s.backup_interval_hours = 24
+    s.backup_keep = 30
+    s.project_sync_enabled = False
 
     from app.db import init_db
     from app.main import app
@@ -47,6 +65,3 @@ def client():
         Path(backup_dir).rmdir()
     except OSError:
         pass
-
-
-from fastapi.testclient import TestClient