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

Dockerfile

@@ -1,10 +1,3 @@
-FROM golang:1.25-bookworm AS backend
-WORKDIR /src
-COPY backend/go.mod backend/go.sum ./
-RUN go mod download
-COPY backend/ .
-RUN CGO_ENABLED=0 go build -o /server ./cmd/server/
-
 FROM node:22-alpine AS frontend
 WORKDIR /src
 COPY frontend/package.json frontend/package-lock.json ./
@@ -12,6 +5,14 @@ RUN npm ci
 COPY frontend/ .
 RUN npm run build
 
+FROM golang:1.24-bookworm AS backend
+WORKDIR /src
+COPY backend/go.mod backend/go.sum ./
+RUN go mod download
+COPY --from=frontend /src/dist /src/embed/dist
+COPY backend/ .
+RUN CGO_ENABLED=0 go build -o /server ./cmd/server/
+
 FROM debian:bookworm-slim
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \