Pin versions, add resource limits, and harden config

- Pin all Docker image tags (mysql 8.0.40, python 3.12.13-slim, node 20.20.1-alpine, nginx 1.29.6-alpine)
- Pin all frontend npm dependencies to exact versions (remove ^ ranges)
- Add mem_limit and cpus resource limits to all three containers
- Add non-root appuser to backend Dockerfile
- Migrate JWT from python-jose to PyJWT
- Remove default admin_password in config.py — must be explicitly set in .env
- Add DOCS_ENABLED flag to config and .env.example (default false)
- Add indexes on session_date, is_active, event_type in session models
- Add limit/offset pagination to all log endpoints

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/Dockerfile

@@ -1,5 +1,5 @@
 # Stage 1: Build Vue.js app
-FROM node:20-alpine AS builder
+FROM node:20.20.1-alpine AS builder
 
 WORKDIR /app
 
@@ -10,7 +10,7 @@ COPY . .
 RUN npm run build
 
 # Stage 2: Serve with nginx
-FROM nginx:alpine
+FROM nginx:1.29.6-alpine
 
 COPY --from=builder /app/dist /usr/share/nginx/html
 COPY nginx.conf /etc/nginx/conf.d/default.conf