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/app/replay/models.py

@@ -0,0 +1,52 @@
+"""Value objects for conversation replay."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from enum import Enum
+
+
+class StepType(str, Enum):
+    """Types of steps in a conversation replay."""
+
+    user_message = "user_message"
+    supervisor_routing = "supervisor_routing"
+    tool_call = "tool_call"
+    tool_result = "tool_result"
+    agent_response = "agent_response"
+    interrupt = "interrupt"
+
+
+@dataclass(frozen=True)
+class ReplayStep:
+    """A single step in a conversation replay."""
+
+    step: int
+    type: StepType
+    timestamp: str
+    content: str = ""
+    agent: str | None = None
+    tool: str | None = None
+    params: dict | None = None
+    result: dict | None = None
+    reasoning: str | None = None
+    tokens: int | None = None
+    duration_ms: int | None = None
+
+    def __post_init__(self) -> None:
+        # Store params as a frozen copy to prevent mutation from the outside
+        if self.params is not None:
+            object.__setattr__(self, "params", dict(self.params))
+        if self.result is not None:
+            object.__setattr__(self, "result", dict(self.result))
+
+
+@dataclass(frozen=True)
+class ReplayPage:
+    """A paginated page of replay steps for a conversation thread."""
+
+    thread_id: str
+    total_steps: int
+    page: int
+    per_page: int
+    steps: tuple[ReplayStep, ...]