fix(security): patch 4 critical vulnerabilities

[Frankshen923]

Summary

Security audit identified 4 critical vulnerabilities. This PR patches all of them with minimal, focused changes (7 files, +63/-23 lines).

1. Unauthenticated API Key Generation (gateway.py)

POST /gateway/agents/{agent_id}/api-key had no authentication. Added Depends(get_current_user) + creator/admin role check using existing check_agent_access().

2. API Keys Stored in Plaintext (enterprise.py, agents.py, task_executor.py)

• LLM API keys: Now encrypted using existing encrypt_data()/decrypt_data() (AES-256-CBC)
• Agent API keys: Now hashed with SHA-256 (consistent with the create flow in gateway.py)

3. Default JWT Secrets Accepted in Production (main.py)

Added startup check: application refuses to boot if SECRET_KEY or JWT_SECRET_KEY contains "change-me" unless DEBUG=true.

4. Multi-Tenant Isolation Gaps (permissions.py, plaza.py, task_executor.py)

• check_agent_access(): Added tenant_id validation for non-admin users
• Plaza API: list_posts and plaza_stats now require authentication and enforce tenant from JWT
• LLM model lookup: Added tenant_id filter to prevent cross-tenant model access

Test Plan

• POST /gateway/agents/{id}/api-key returns 401 without auth token
• LLM API keys encrypted in database (api_key_encrypted column is ciphertext)
• App refuses to start with default SECRET_KEY in non-DEBUG mode
• Cross-tenant agent access returns 403
• GET /plaza/posts without auth returns 401; with auth returns only own tenant's posts
• LLM model lookup respects tenant boundaries

Notes

• All fixes use existing infrastructure (encrypt_data, check_agent_access, get_current_user) — no new dependencies
• Backward compatible: existing encrypted keys will work after migration (plaintext keys need re-encryption)
• Discovered during enterprise deployment evaluation

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