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/models.py

@@ -0,0 +1,38 @@
+"""Value objects for analytics dashboard."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+
+@dataclass(frozen=True)
+class AgentUsage:
+    """Agent usage statistics within a time range."""
+
+    agent: str
+    count: int
+    percentage: float
+
+
+@dataclass(frozen=True)
+class InterruptStats:
+    """Interrupt approval/rejection statistics within a time range."""
+
+    total: int = 0
+    approved: int = 0
+    rejected: int = 0
+    expired: int = 0
+
+
+@dataclass(frozen=True)
+class AnalyticsResult:
+    """Full analytics result for a given time range."""
+
+    range: str
+    total_conversations: int
+    resolution_rate: float
+    escalation_rate: float
+    avg_turns_per_conversation: float
+    avg_cost_per_conversation_usd: float
+    agent_usage: tuple[AgentUsage, ...]
+    interrupt_stats: InterruptStats