feat: complete phase 4 -- conversation replay API + analytics dashboard

- Replay models: StepType enum, ReplayStep, ReplayPage frozen dataclasses
- Checkpoint transformer: PostgresSaver JSONB -> structured timeline steps
- Replay API: GET /api/conversations (paginated), GET /api/replay/{thread_id}
- Analytics models: AgentUsage, InterruptStats, AnalyticsResult
- Analytics event recorder: Protocol + PostgresAnalyticsRecorder + NoOp
- Analytics queries: resolution_rate, agent_usage, escalation_rate, cost, interrupts
- Analytics API: GET /api/analytics?range=Xd with envelope response
- DB migration: analytics_events table + conversations column additions
- 74 new tests, 399 total passing, 92.87% coverage

backend/tests/unit/replay/test_transformer.py

@@ -0,0 +1,155 @@
+"""Unit tests for app.replay.transformer."""
+
+from __future__ import annotations
+
+import pytest
+
+pytestmark = pytest.mark.unit
+
+
+def _make_row(messages: list[dict], metadata: dict | None = None) -> dict:
+    """Helper to build a checkpoint row with the given messages."""
+    return {
+        "thread_id": "thread-abc",
+        "checkpoint_id": "cp-001",
+        "checkpoint": {"channel_values": {"messages": messages}},
+        "metadata": metadata or {},
+    }
+
+
+class TestTransformCheckpoints:
+    def test_empty_rows_returns_empty_list(self) -> None:
+        from app.replay.transformer import transform_checkpoints
+
+        result = transform_checkpoints([])
+        assert result == []
+
+    def test_human_message_produces_user_message_step(self) -> None:
+        from app.replay.models import StepType
+        from app.replay.transformer import transform_checkpoints
+
+        rows = [_make_row([{"type": "human", "content": "Hello, I need help"}])]
+        steps = transform_checkpoints(rows)
+        assert len(steps) == 1
+        assert steps[0].type == StepType.user_message
+        assert steps[0].content == "Hello, I need help"
+        assert steps[0].step == 1
+
+    def test_ai_message_with_content_produces_agent_response(self) -> None:
+        from app.replay.models import StepType
+        from app.replay.transformer import transform_checkpoints
+
+        rows = [
+            _make_row(
+                [{"type": "ai", "content": "I can help you with that.", "tool_calls": []}],
+                metadata={"writes": {"some_agent": "response"}},
+            )
+        ]
+        steps = transform_checkpoints(rows)
+        assert len(steps) == 1
+        assert steps[0].type == StepType.agent_response
+        assert steps[0].content == "I can help you with that."
+
+    def test_ai_message_with_tool_calls_produces_tool_call_step(self) -> None:
+        from app.replay.models import StepType
+        from app.replay.transformer import transform_checkpoints
+
+        rows = [
+            _make_row(
+                [
+                    {
+                        "type": "ai",
+                        "content": "",
+                        "tool_calls": [
+                            {
+                                "name": "get_order_status",
+                                "args": {"order_id": "ORD-123"},
+                                "id": "call_abc",
+                            }
+                        ],
+                    }
+                ]
+            )
+        ]
+        steps = transform_checkpoints(rows)
+        assert len(steps) == 1
+        assert steps[0].type == StepType.tool_call
+        assert steps[0].tool == "get_order_status"
+        assert steps[0].params == {"order_id": "ORD-123"}
+
+    def test_tool_message_produces_tool_result_step(self) -> None:
+        from app.replay.models import StepType
+        from app.replay.transformer import transform_checkpoints
+
+        rows = [
+            _make_row(
+                [
+                    {
+                        "type": "tool",
+                        "content": '{"status": "shipped"}',
+                        "name": "get_order_status",
+                    }
+                ]
+            )
+        ]
+        steps = transform_checkpoints(rows)
+        assert len(steps) == 1
+        assert steps[0].type == StepType.tool_result
+        assert steps[0].tool == "get_order_status"
+
+    def test_multiple_messages_sequential_steps(self) -> None:
+        from app.replay.transformer import transform_checkpoints
+
+        rows = [
+            _make_row(
+                [
+                    {"type": "human", "content": "Help"},
+                    {"type": "ai", "content": "Sure!", "tool_calls": []},
+                ]
+            )
+        ]
+        steps = transform_checkpoints(rows)
+        assert len(steps) == 2
+        assert steps[0].step == 1
+        assert steps[1].step == 2
+
+    def test_unknown_message_type_skipped(self) -> None:
+        from app.replay.transformer import transform_checkpoints
+
+        rows = [_make_row([{"type": "unknown_type", "content": "test"}])]
+        steps = transform_checkpoints(rows)
+        # Should not crash; unknown types may be skipped
+        assert isinstance(steps, list)
+
+    def test_row_missing_checkpoint_skipped(self) -> None:
+        from app.replay.transformer import transform_checkpoints
+
+        rows = [{"thread_id": "t1", "checkpoint_id": "cp1", "checkpoint": None, "metadata": {}}]
+        steps = transform_checkpoints(rows)
+        assert isinstance(steps, list)
+
+    def test_row_missing_messages_key_skipped(self) -> None:
+        from app.replay.transformer import transform_checkpoints
+
+        rows = [{"thread_id": "t1", "checkpoint_id": "cp1", "checkpoint": {}, "metadata": {}}]
+        steps = transform_checkpoints(rows)
+        assert isinstance(steps, list)
+
+    def test_multiple_rows_steps_are_continuous(self) -> None:
+        from app.replay.transformer import transform_checkpoints
+
+        rows = [
+            _make_row([{"type": "human", "content": "Q1"}]),
+            _make_row([{"type": "ai", "content": "A1", "tool_calls": []}]),
+        ]
+        steps = transform_checkpoints(rows)
+        assert len(steps) == 2
+        assert steps[0].step == 1
+        assert steps[1].step == 2
+
+    def test_timestamps_are_strings(self) -> None:
+        from app.replay.transformer import transform_checkpoints
+
+        rows = [_make_row([{"type": "human", "content": "Hi"}])]
+        steps = transform_checkpoints(rows)
+        assert isinstance(steps[0].timestamp, str)