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_models.py

@@ -0,0 +1,134 @@
+"""Unit tests for app.replay.models."""
+
+from __future__ import annotations
+
+import pytest
+
+pytestmark = pytest.mark.unit
+
+
+class TestStepType:
+    def test_all_step_types_exist(self) -> None:
+        from app.replay.models import StepType
+
+        assert StepType.user_message
+        assert StepType.supervisor_routing
+        assert StepType.tool_call
+        assert StepType.tool_result
+        assert StepType.agent_response
+        assert StepType.interrupt
+
+    def test_step_type_values(self) -> None:
+        from app.replay.models import StepType
+
+        assert StepType.user_message.value == "user_message"
+        assert StepType.tool_call.value == "tool_call"
+        assert StepType.agent_response.value == "agent_response"
+
+
+class TestReplayStep:
+    def test_minimal_replay_step(self) -> None:
+        from app.replay.models import ReplayStep, StepType
+
+        step = ReplayStep(step=1, type=StepType.user_message, timestamp="2026-01-01T00:00:00Z")
+        assert step.step == 1
+        assert step.type == StepType.user_message
+        assert step.timestamp == "2026-01-01T00:00:00Z"
+        assert step.content == ""
+        assert step.agent is None
+        assert step.tool is None
+        assert step.params is None
+        assert step.result is None
+        assert step.reasoning is None
+        assert step.tokens is None
+        assert step.duration_ms is None
+
+    def test_full_replay_step(self) -> None:
+        from app.replay.models import ReplayStep, StepType
+
+        step = ReplayStep(
+            step=2,
+            type=StepType.tool_call,
+            timestamp="2026-01-01T00:00:01Z",
+            content="calling get_order",
+            agent="order_agent",
+            tool="get_order_status",
+            params={"order_id": "ORD-123"},
+            result={"status": "shipped"},
+            reasoning="user asked about order",
+            tokens=50,
+            duration_ms=200,
+        )
+        assert step.step == 2
+        assert step.agent == "order_agent"
+        assert step.tool == "get_order_status"
+        assert step.params == {"order_id": "ORD-123"}
+        assert step.tokens == 50
+
+    def test_replay_step_is_frozen(self) -> None:
+        from app.replay.models import ReplayStep, StepType
+
+        step = ReplayStep(step=1, type=StepType.user_message, timestamp="2026-01-01T00:00:00Z")
+        with pytest.raises((AttributeError, TypeError)):
+            step.step = 99  # type: ignore[misc]
+
+    def test_replay_step_params_is_immutable_copy(self) -> None:
+        from app.replay.models import ReplayStep, StepType
+
+        params = {"key": "value"}
+        step = ReplayStep(
+            step=1,
+            type=StepType.tool_call,
+            timestamp="2026-01-01T00:00:00Z",
+            params=params,
+        )
+        # Modifying original dict should not affect step
+        params["new_key"] = "new_value"
+        assert "new_key" not in (step.params or {})
+
+
+class TestReplayPage:
+    def test_replay_page_construction(self) -> None:
+        from app.replay.models import ReplayPage, ReplayStep, StepType
+
+        steps = (
+            ReplayStep(step=1, type=StepType.user_message, timestamp="2026-01-01T00:00:00Z"),
+            ReplayStep(step=2, type=StepType.agent_response, timestamp="2026-01-01T00:00:01Z"),
+        )
+        page = ReplayPage(
+            thread_id="thread-123",
+            total_steps=2,
+            page=1,
+            per_page=20,
+            steps=steps,
+        )
+        assert page.thread_id == "thread-123"
+        assert page.total_steps == 2
+        assert page.page == 1
+        assert page.per_page == 20
+        assert len(page.steps) == 2
+
+    def test_replay_page_is_frozen(self) -> None:
+        from app.replay.models import ReplayPage
+
+        page = ReplayPage(
+            thread_id="t1",
+            total_steps=0,
+            page=1,
+            per_page=20,
+            steps=(),
+        )
+        with pytest.raises((AttributeError, TypeError)):
+            page.page = 2  # type: ignore[misc]
+
+    def test_replay_page_empty_steps(self) -> None:
+        from app.replay.models import ReplayPage
+
+        page = ReplayPage(
+            thread_id="t1",
+            total_steps=0,
+            page=1,
+            per_page=20,
+            steps=(),
+        )
+        assert page.steps == ()