fix: resolve 20 security vulnerabilities from comprehensive audit

Critical (1-3):
- Replace hardcoded admin credentials with secure auto-generation (seed.py)
- Enforce SECRET_KEY configuration, fail in production if missing (config.py)
- Add Zip Slip and Zip Bomb protection to all ZIP import services

High/Medium (4-9):
- Add 50MB file size limit and extension whitelist to evidence uploads
- Configure CORS origins via environment variable instead of hardcoded
- Migrate JWT storage from localStorage to HttpOnly cookies (frontend+backend)
- Add rate limiting (5/min) on login endpoint via slowapi
- Replace generic dict payloads with Pydantic schemas (mass assignment)

Medium (10-17):
- Check is_active on login to prevent disabled users from authenticating
- Sanitize exception messages in API responses (system, data_sources)
- Escape LIKE wildcards in all ilike search filters across 8 routers
- Run Docker container as non-root user (appuser)
- Make MINIO_SECURE configurable via environment variable
- Add password complexity policy (12+ chars, upper/lower/digit/special)
- Implement JWT token revocation via in-memory blacklist + reduce TTL to 15min
- Replace xml.etree with defusedxml to prevent Billion Laughs attacks

Low (18-20):
- Add security headers to Nginx (CSP, X-Frame-Options, HSTS-ready, etc.)
- Disable Swagger UI/ReDoc/OpenAPI in production
- Restrict /health endpoint to internal networks via Nginx ACL

Also: rewrite install.sh as interactive wizard for guided deployment,
fix test-from-template validation error (technique_id UUID vs MITRE ID)

backend/Dockerfile

@@ -19,6 +19,12 @@ COPY . .
 # Make entrypoints executable
 RUN chmod +x /app/entrypoint.sh /app/entrypoint.prod.sh
 
+# Create a non-root user and give it ownership of /app
+RUN adduser --disabled-password --gecos '' --uid 1001 appuser \
+    && chown -R appuser:appuser /app
+
+USER appuser
+
 # Expose port
 EXPOSE 8000