fix(quota): exempt admins from agent quota and add per-role limits (#274)

[vybe]

Summary

• Fix: Admin users are now exempt from agent creation quotas (was incorrectly blocking admins)
• Feature: Per-role quota configuration via Settings UI (admin=unlimited, creator=10, operator=3, user=1 defaults)
• Improvement: Quota exceeded error now shows current/max agent counts

Changes

• src/backend/services/settings_service.py — Added get_agent_quota_for_role() helper with per-role defaults and legacy fallback
• src/backend/services/agent_service/crud.py — Use per-role quota check instead of global setting
• src/backend/services/agent_service/deploy.py — Same per-role quota check for deploy-local path
• src/backend/routers/settings.py — Added GET/PUT /api/settings/agent-quotas endpoints
• src/frontend/src/views/Settings.vue — Added Agent Quotas section with per-role inputs
• src/frontend/src/components/CreateAgentModal.vue — Better QUOTA_EXCEEDED error display
• tests/test_agent_quota.py — Updated tests for admin exemption and per-role configuration

Test Plan

• All 13 quota tests pass (pytest test_agent_quota.py -v)
• Admin user can create agents past any quota limit
• GET /api/settings/agent-quotas returns per-role config with defaults
• PUT /api/settings/agent-quotas persists per-role limits
• Negative quota values rejected with 400
• Legacy max_agents_per_user still works as fallback
• Redeploy bypass still works

Closes #274

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