fix: code quality, memory safety, and install improvements

Critical fixes:
- Fix Dockerfile: reorder stages so frontend assets embed into Go binary
- Fix Go version 1.25 (nonexistent) to 1.24 across Dockerfile, go.mod, CI
- Add graceful game server shutdown on SIGTERM/SIGINT
- Order startup tasks: updates complete before auto-start
- Fix TOCTOU race in UpdateSettings with atomic Update() method

Security:
- Add optional AUTH_TOKEN bearer auth middleware on API/WS routes
- Fix path traversal in DeleteMod using filepath.Rel instead of HasPrefix
- Add input validation for IPPort, ServerParameters, ScheduledUpdate

Memory safety:
- Cap RPT buffer allocation to 64KB to prevent OOM on large logs
- Cap GetLog file read to 10MB
- Fix context cancel leak in SteamCmdManager.run()
- Remove data-raced cancel field in steamcmd.go
- Atomic file writes (write-temp-then-rename) across all managers

Reliability:
- Log save errors in ProcessManager.Stop()
- Atomic file writes prevent corruption on crash

Tests:
- Add mod_manager_test.go (12 tests: ListWorkshopMods, ListLocalMods,
  BuildUsageMap, RemoveMod, dirSize)
- Add scheduler_test.go (6 tests: Start/Stop, Refresh with empty,
  invalid, valid, and replaced cron expressions)
- Add TestRestart to server_process_test.go

CI/Docs:
- Add -race flag to go test in CI and Makefile
- Add npm lint step to CI
- Add Go/npm module caching to CI
- Update README: prerequisites, AUTH_TOKEN/GIN_MODE/SERVERS_DIR docs,
  fix manual quickstart to use make build

.gitea/workflows/ci.yml

@@ -12,12 +12,17 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions/setup-go@v5
         with:
-          go-version: "1.25"
+          go-version: "1.24"
+          cache: true
+          cache-dependency-path: backend/go.sum
       - uses: actions/setup-node@v4
         with:
           node-version: "22"
+          cache: "npm"
+          cache-dependency-path: frontend/package-lock.json
       - run: cd frontend && npm ci && npm run build
       - run: mkdir -p backend/embed/dist && cp -r frontend/dist/. backend/embed/dist/
       - run: cd backend && go build ./...
-      - run: cd backend && go test ./...
+      - run: cd backend && go test -race ./...
+      - run: cd frontend && npm run lint
       - run: cd frontend && npm test