feat(agent-chat): 035-agent-chat-context — контекст, guardrails, tools, database discovery

== User stories ==
US1: Контекст с дашборда/датасета → /agent с URL params
US2: Guardrails card — env badge, 7 risk tones, countdown, permission_denied
US3: Tools optimization — retry, timeout, summarise, RBAC + context affinity

== Backend ==
- _context.py (NEW): UIContext validation (7 checks)
- _tool_filter.py (NEW): RBAC + context affinity pipeline
- _confirmation.py: build_confirmation_contract_v2, permission_denied_payload
- tools.py: superset_list_databases, retry/summarise/timeout wrappers
- app.py: _inject_uicontext, _inject_env_id_into_tools, database
  prefetch в runtime context
- _persistence.py: prefetch_databases()
- agent_superset_explore.py: GET /databases endpoint
- _llm_async_http.py, _persistence.py: fix double /v1 в LLM URL
  (LM Studio Unexpected endpoint)

== Frontend ==
- AgentChatModel.svelte.ts: 5 atoms, 3 actions, countdown, context
- AgentChat.svelte: production banner, process steps, debug panel
- ConfirmationCard.svelte: 7 risk tones, permission_denied, countdown
- ToolCallCard.svelte: retrying/timeout/cancelled states
- StreamProcessor.svelte.ts: tool_retry, timeout, permission_denied
- TopNavbar: sparkles icon + Ассистент
- sidebarNavigation: AI section
- DashboardHeader, datasets/+page: contextual AI buttons
- Icon: sparkles, brain, cpu icons
- tailwind: assistant category colors
- i18n: en/ru nav keys

== Tests ==
- 159 backend agent tests (+16 US3: retry, timeout, summarise, contracts)
- 2544 frontend tests (+11 model + component tests)
- 15 JSON fixtures (10 API + 5 model)

== Specs ==
- specs/035-agent-chat-context/: spec, UX, plan, tasks, research,
  data-model, contracts, quickstart, traceability, fixtures, checklists

Closes #035

frontend/tailwind.config.js

@@ -108,6 +108,7 @@ export default {
           tools:        { from: '#f1f5f9', to: '#e2e8f0', text: '#334155', ring: '#94a3b8' },   // slate→surface
           admin:        { from: '#ffe4e6', to: '#fecdd3', text: '#be123c', ring: '#fda4af' },   // rose→destructive
           maintenance:  { from: '#ffedd5', to: '#fed7aa', text: '#c2410c', ring: '#fdba74' },   // orange→warning
+          assistant:    { from: '#f3e8ff', to: '#e9d5ff', text: '#6b21a8', ring: '#d8b4fe' },   // violet→purple
         },
       },
       width: {