feat: complete phase 5 -- error hardening, frontend, Docker, demo, docs

Backend:
- ConversationTracker: Protocol + PostgresConversationTracker for lifecycle tracking
- Error handler: ErrorCategory enum, classify_error(), with_retry() exponential backoff
- Wire PostgresAnalyticsRecorder + ConversationTracker into ws_handler
- Rate limiting (10 msg/10s per thread), edge case hardening
- Health endpoint GET /api/health, version 0.5.0
- Demo seed data script + sample OpenAPI spec

Frontend (all new):
- React Router with NavBar (Chat / Replay / Dashboard / Review)
- ReplayListPage + ReplayPage with ReplayTimeline component
- DashboardPage with MetricCard, range selector, zero-state
- ReviewPage for OpenAPI classification review
- ErrorBanner for WebSocket disconnect handling
- API client (api.ts) with typed fetch wrappers

Infrastructure:
- Frontend Dockerfile (multi-stage node -> nginx)
- nginx.conf with SPA routing + API/WS proxy
- docker-compose.yml with frontend service + healthchecks
- .env.example files (root + backend)

Documentation:
- README.md with quick start and architecture
- Agent configuration guide
- OpenAPI import guide
- Deployment guide
- Demo script

48 new tests, 449 total passing, 92.87% coverage

backend/app/conversation_tracker.py

@@ -0,0 +1,135 @@
+"""Conversation tracker -- Protocol and implementations for tracking conversation state."""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING, Protocol, runtime_checkable
+
+if TYPE_CHECKING:
+    from psycopg_pool import AsyncConnectionPool
+
+_ENSURE_SQL = """
+INSERT INTO conversations
+    (thread_id, started_at, last_activity)
+VALUES
+    (%(thread_id)s, NOW(), NOW())
+ON CONFLICT (thread_id) DO NOTHING
+"""
+
+_RECORD_TURN_SQL = """
+UPDATE conversations
+SET
+    turn_count = turn_count + 1,
+    agents_used = CASE
+        WHEN %(agent_name)s IS NOT NULL AND NOT (agents_used @> ARRAY[%(agent_name)s]::text[])
+        THEN agents_used || ARRAY[%(agent_name)s]::text[]
+        ELSE agents_used
+    END,
+    total_tokens = total_tokens + %(tokens)s,
+    total_cost_usd = total_cost_usd + %(cost)s,
+    last_activity = NOW()
+WHERE thread_id = %(thread_id)s
+"""
+
+_RESOLVE_SQL = """
+UPDATE conversations
+SET
+    resolution_type = %(resolution_type)s,
+    ended_at = NOW()
+WHERE thread_id = %(thread_id)s
+"""
+
+
+@runtime_checkable
+class ConversationTrackerProtocol(Protocol):
+    """Protocol for tracking conversation lifecycle and metrics."""
+
+    async def ensure_conversation(self, pool: AsyncConnectionPool, thread_id: str) -> None:
+        """Create conversation row if it does not already exist."""
+        ...
+
+    async def record_turn(
+        self,
+        pool: AsyncConnectionPool,
+        thread_id: str,
+        agent_name: str | None,
+        tokens: int,
+        cost: float,
+    ) -> None:
+        """Increment turn count and update aggregated metrics."""
+        ...
+
+    async def resolve(
+        self,
+        pool: AsyncConnectionPool,
+        thread_id: str,
+        resolution_type: str,
+    ) -> None:
+        """Mark conversation as resolved with a resolution type."""
+        ...
+
+
+class NoOpConversationTracker:
+    """No-op implementation -- used in tests or when DB is unavailable."""
+
+    async def ensure_conversation(self, pool: AsyncConnectionPool, thread_id: str) -> None:
+        """Do nothing."""
+
+    async def record_turn(
+        self,
+        pool: AsyncConnectionPool,
+        thread_id: str,
+        agent_name: str | None,
+        tokens: int,
+        cost: float,
+    ) -> None:
+        """Do nothing."""
+
+    async def resolve(
+        self,
+        pool: AsyncConnectionPool,
+        thread_id: str,
+        resolution_type: str,
+    ) -> None:
+        """Do nothing."""
+
+
+class PostgresConversationTracker:
+    """Postgres-backed conversation tracker."""
+
+    async def ensure_conversation(self, pool: AsyncConnectionPool, thread_id: str) -> None:
+        """Insert conversation row; do nothing if already exists (ON CONFLICT DO NOTHING)."""
+        params = {"thread_id": thread_id}
+        async with pool.connection() as conn:
+            await conn.execute(_ENSURE_SQL, params)
+
+    async def record_turn(
+        self,
+        pool: AsyncConnectionPool,
+        thread_id: str,
+        agent_name: str | None,
+        tokens: int,
+        cost: float,
+    ) -> None:
+        """Increment turn count, append agent if new, update token/cost totals."""
+        params = {
+            "thread_id": thread_id,
+            "agent_name": agent_name,
+            "tokens": tokens,
+            "cost": cost,
+        }
+        async with pool.connection() as conn:
+            await conn.execute(_RECORD_TURN_SQL, params)
+
+    async def resolve(
+        self,
+        pool: AsyncConnectionPool,
+        thread_id: str,
+        resolution_type: str,
+    ) -> None:
+        """Set resolution_type and ended_at on the conversation row."""
+        params = {
+            "thread_id": thread_id,
+            "resolution_type": resolution_type,
+        }
+        async with pool.connection() as conn:
+            await conn.execute(_RESOLVE_SQL, params)