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

backend/tests/test_agent_timeout.py

@@ -0,0 +1,87 @@
+# #region Test.AgentChat.ToolTimeout [C:3] [TYPE Module] [SEMANTICS test,agent,tools,timeout]
+# @BRIEF Contract tests for _execute_with_timeout — configurable timeout wrapper.
+# @RELATION BINDS_TO -> [AgentChat.Tools.Timeout]
+# @TEST_EDGE: complete_under_timeout -> Tool completes normally, returns result
+# @TEST_EDGE: read_tool_timeout -> Read tool exceeds timeout, raises TimeoutError
+# @TEST_EDGE: write_tool_timeout -> Write tool exceeds timeout, raises TimeoutError
+
+import asyncio
+from unittest.mock import AsyncMock, patch
+
+import pytest
+
+
+# ── Tests ───────────────────────────────────────────────────────────
+
+
+# #region test_completes_under_timeout [C:2] [TYPE Function]
+# @BRIEF GIVEN a tool that returns quickly WHEN _execute_with_timeout is called with a 30s timeout THEN the result is returned normally.
+@pytest.mark.asyncio
+async def test_completes_under_timeout():
+    """Tool returns its result before the timeout expires."""
+    from src.agent.tools import _execute_with_timeout
+
+    expected = {"status": "ok", "data": [1, 2, 3]}
+    fast_fn = AsyncMock(return_value=expected)
+
+    with patch("src.agent.tools.logger.explore") as mock_explore:
+        result = await _execute_with_timeout("fast_tool", fast_fn, is_write=False, timeout_s=30)
+
+    assert result == expected
+    fast_fn.assert_called_once()
+    mock_explore.assert_not_called()
+# #endregion test_completes_under_timeout
+
+
+# #region test_read_tool_timeout [C:2] [TYPE Function]
+# @BRIEF GIVEN a read tool that exceeds the timeout WHEN _execute_with_timeout is called THEN TimeoutError is raised and logger.explore is invoked.
+@pytest.mark.asyncio
+async def test_read_tool_timeout():
+    """Read tool that sleeps longer than timeout_s must raise TimeoutError."""
+    from src.agent.tools import _execute_with_timeout
+
+    async def slow_read():
+        await asyncio.sleep(0.3)
+        return "never_reached"
+
+    with patch("src.agent.tools.logger.explore") as mock_explore:
+        with pytest.raises(TimeoutError):
+            await _execute_with_timeout("slow_read", slow_read, is_write=False, timeout_s=0.05)
+
+    mock_explore.assert_called_once()
+    call_args = mock_explore.call_args
+    assert call_args[0][0] == "Tool timeout"
+    payload = call_args[1]["payload"]
+    assert payload["tool"] == "slow_read"
+    assert payload["timeout_s"] == 0.05
+    assert payload["is_write"] is False
+    assert call_args[1]["extra"]["src"] == "AgentChat.Tools.Timeout"
+# #endregion test_read_tool_timeout
+
+
+# #region test_write_tool_timeout [C:2] [TYPE Function]
+# @BRIEF GIVEN a write tool that exceeds the timeout WHEN _execute_with_timeout is called THEN TimeoutError is raised with is_write=True logged.
+@pytest.mark.asyncio
+async def test_write_tool_timeout():
+    """Write tool that sleeps longer than timeout_s must raise TimeoutError."""
+    from src.agent.tools import _execute_with_timeout
+
+    async def slow_write():
+        await asyncio.sleep(0.3)
+        return "never_reached"
+
+    with patch("src.agent.tools.logger.explore") as mock_explore:
+        with pytest.raises(TimeoutError):
+            await _execute_with_timeout("slow_write", slow_write, is_write=True, timeout_s=0.05)
+
+    mock_explore.assert_called_once()
+    call_args = mock_explore.call_args
+    assert call_args[0][0] == "Tool timeout"
+    payload = call_args[1]["payload"]
+    assert payload["tool"] == "slow_write"
+    assert payload["timeout_s"] == 0.05
+    assert payload["is_write"] is True
+    assert call_args[1]["extra"]["src"] == "AgentChat.Tools.Timeout"
+# #endregion test_write_tool_timeout
+
+# #endregion Test.AgentChat.ToolTimeout