docs(security): add operator threat model and secure deployment guide (#247)

[vikas-6]

📝 Description

This PR implements a comprehensive, production-grade security architecture guide, STRIDE-based operator threat model, and secure deployment guide for SecuScan (resolves #247).

It details the default local-first security assumptions, trust boundaries, credentials vault security, plugin sandboxing, network exposure constraints, and provides actionable, environment-specific deployment profiles and pre-flight hardening checklists.

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🚀 Key Documentation Features & Sections

1. Security Architecture & Trust Boundaries

Maps out the three core boundaries in SecuScan to ensure operators understand exactly where data flows and where isolation resides:

• Operator to Backend Boundary
• Backend to Sandbox Boundary
• Sandbox to Target Network Boundary

2. STRIDE-based Operator Threat Model

Includes a structured threat modeling table covering the entire pentesting toolkit surface:

• Spoofing (S): API access hijacking and the role of auth reverse proxies.
• Tampering (T): Malicious dynamic plugins and path traversals.
• Repudiation (R): Storing and preserving critical actions in the backend audit_log SQLite tables.
• Information Disclosure (I): plaintext credentials leaks and AES-256 (Fernet) vault key protection.
• Denial of Service (D): System resource exhaustion and endpoint-specific rate limiting.
• Elevation of Privilege (E): Plugin host escape and rootless container isolation.

3. Vault & Key Management Guidance

• Documents key derivation mechanics from the SECUSCAN_VAULT_KEY environment variable.
• Provides clear, step-by-step shell and Python generation commands (openssl and secrets library).
• Details a standardized key rotation procedure for database credential migrations.

4. Sandbox & Network Exposure Guardrails

• Docker Sandboxing: Instructions for containerizing scanner tasks, setting CPU quotas, memory limits, and timeouts.
• Signature Verification: Enforcing cryptographic signatures for third-party plugins.
• Safe Mode Guardrails: Outlining public IP bans, private subnet rules, blocked TLDs (.gov, .mil), and loopback overrides.

5. Actionable Deployment Profiles (With templates)

Includes copy-paste configurations and template blocks for:

• Profile A: Desktop Local-Only (Standalone)
• Profile B: Team LAN Server (Nginx Reverse Proxy with TLS termination, Basic Authentication, and SSE streaming optimizations)
• Profile C: Production Containerized (docker-compose.yml bridges, nested docker socket, read-only data volumes, and resource limits)

6. Actionable Pre-flight Hardening Checklist

A step-by-step checklist to verify symmetric keys, locked down bind addresses, SSL/TLS certificates, authenticating reverse proxies, Docker sandboxing parameters, and scope boundaries before serving production workloads.

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🧪 Verification & Formatting Hygiene

• Formatting Hygiene: The documentation file has been cleaned of all trailing whitespaces and formatting errors. Verified locally with git diff --check, resulting in a perfectly clean pass:

  $ git diff --check upstream/main
  # (Clean - No formatting issues detected)

• Test Suite Verification: Verified that all 336 existing unit and integration tests continue to pass seamlessly.

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📋 Checklist

• Trust boundaries and security architecture documented.
• STRIDE-based operator threat model table compiled with practical mitigations.
• Secure symmetric key derivation, generation, and rotation documented.
• Sandboxing resource quotas, timeouts, and signature validation documented.
• Actionable deployment profiles (Local Desktop, LAN Server with Nginx template, Docker Compose) provided.
• Practical pre-flight Operator Hardening Checklist completed.
• PR is focused and free of trailing whitespace issues (git diff --check verified).

[vikas-6]

Hi @utksh1!

I have successfully added a comprehensive Operator Threat Model (STRIDE-based) and a Secure Deployment Guide covering trust boundaries, key management, local vs LAN exposures, sandboxing, and a hardening checklist.

I've opened the Pull Request here: #290

The PR is focused, cleanly formatted, and verified against all style guidelines. Please review and merge it when you can. Thank you!