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/analytics/event_recorder.py

@@ -0,0 +1,95 @@
+"""Analytics event recorder -- Protocol and implementations."""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING, Any, Protocol, runtime_checkable
+
+if TYPE_CHECKING:
+    from psycopg_pool import AsyncConnectionPool
+
+_INSERT_SQL = """
+INSERT INTO analytics_events
+    (thread_id, event_type, agent_name, tool_name, tokens_used, cost_usd,
+     duration_ms, success, error_message, metadata)
+VALUES
+    (%(thread_id)s, %(event_type)s, %(agent_name)s, %(tool_name)s,
+     %(tokens_used)s, %(cost_usd)s, %(duration_ms)s, %(success)s,
+     %(error_message)s, %(metadata)s)
+"""
+
+
+@runtime_checkable
+class AnalyticsRecorder(Protocol):
+    """Protocol for recording analytics events."""
+
+    async def record(
+        self,
+        *,
+        thread_id: str,
+        event_type: str,
+        agent_name: str | None = None,
+        tool_name: str | None = None,
+        tokens_used: int = 0,
+        cost_usd: float = 0.0,
+        duration_ms: int | None = None,
+        success: bool | None = None,
+        error_message: str | None = None,
+        metadata: dict | None = None,
+    ) -> None: ...
+
+
+class NoOpAnalyticsRecorder:
+    """No-op implementation for testing or when the DB is unavailable."""
+
+    async def record(
+        self,
+        *,
+        thread_id: str,
+        event_type: str,
+        agent_name: str | None = None,
+        tool_name: str | None = None,
+        tokens_used: int = 0,
+        cost_usd: float = 0.0,
+        duration_ms: int | None = None,
+        success: bool | None = None,
+        error_message: str | None = None,
+        metadata: dict | None = None,
+    ) -> None:
+        """Do nothing."""
+
+
+class PostgresAnalyticsRecorder:
+    """Postgres-backed analytics recorder -- INSERTs into analytics_events."""
+
+    def __init__(self, pool: AsyncConnectionPool) -> None:
+        self._pool = pool
+
+    async def record(
+        self,
+        *,
+        thread_id: str,
+        event_type: str,
+        agent_name: str | None = None,
+        tool_name: str | None = None,
+        tokens_used: int = 0,
+        cost_usd: float = 0.0,
+        duration_ms: int | None = None,
+        success: bool | None = None,
+        error_message: str | None = None,
+        metadata: dict | None = None,
+    ) -> None:
+        """Insert one analytics event row."""
+        params: dict[str, Any] = {
+            "thread_id": thread_id,
+            "event_type": event_type,
+            "agent_name": agent_name,
+            "tool_name": tool_name,
+            "tokens_used": tokens_used,
+            "cost_usd": cost_usd,
+            "duration_ms": duration_ms,
+            "success": success,
+            "error_message": error_message,
+            "metadata": metadata or {},
+        }
+        async with self._pool.connection() as conn:
+            await conn.execute(_INSERT_SQL, params)