feat: complete phase 1 -- core framework with chat loop, agents, and React UI

Backend:
- FastAPI WebSocket /ws endpoint with streaming via LangGraph astream
- LangGraph Supervisor connecting 3 mock agents (order_lookup, order_actions, fallback)
- YAML Agent Registry with Pydantic validation and immutable configs
- PostgresSaver checkpoint persistence via langgraph-checkpoint-postgres
- Session TTL with 30-min sliding window and interrupt extension
- LLM provider abstraction (Anthropic/OpenAI/Google)
- Token usage + cost tracking callback handler
- Input validation: message size cap, thread_id format, content length
- Security: no hardcoded defaults, startup API key validation, no input reflection

Frontend:
- React 19 + TypeScript + Vite chat UI
- WebSocket hook with reconnect + exponential backoff
- Streaming token display with agent attribution
- Interrupt approval/reject UI for write operations
- Collapsible tool call viewer

Testing:
- 87 unit tests, 87% coverage (exceeds 80% requirement)
- Ruff lint + format clean

Infrastructure:
- Docker Compose (PostgreSQL 16 + backend)
- pyproject.toml with full dependency management

backend/app/main.py

@@ -0,0 +1,83 @@
+"""FastAPI application entry point."""
+
+from __future__ import annotations
+
+import logging
+from contextlib import asynccontextmanager
+from pathlib import Path
+from typing import TYPE_CHECKING
+
+from fastapi import FastAPI, WebSocket, WebSocketDisconnect
+from fastapi.staticfiles import StaticFiles
+
+from app.callbacks import TokenUsageCallbackHandler
+from app.config import Settings
+from app.db import create_checkpointer, create_pool, setup_app_tables
+from app.graph import build_graph
+from app.llm import create_llm
+from app.registry import AgentRegistry
+from app.session_manager import SessionManager
+from app.ws_handler import dispatch_message
+
+if TYPE_CHECKING:
+    from collections.abc import AsyncGenerator
+
+logger = logging.getLogger(__name__)
+
+AGENTS_YAML = Path(__file__).parent.parent / "agents.yaml"
+FRONTEND_DIST = Path(__file__).parent.parent.parent / "frontend" / "dist"
+
+
+@asynccontextmanager
+async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
+    settings = Settings()
+
+    pool = await create_pool(settings)
+    checkpointer = await create_checkpointer(pool)
+    await setup_app_tables(pool)
+
+    registry = AgentRegistry.load(AGENTS_YAML)
+    llm = create_llm(settings)
+    graph = build_graph(registry, llm, checkpointer)
+    session_manager = SessionManager(
+        session_ttl_seconds=settings.session_ttl_minutes * 60,
+    )
+
+    app.state.graph = graph
+    app.state.session_manager = session_manager
+    app.state.settings = settings
+    app.state.pool = pool
+
+    logger.info(
+        "Smart Support started: %d agents loaded, LLM=%s/%s",
+        len(registry),
+        settings.llm_provider,
+        settings.llm_model,
+    )
+
+    yield
+
+    await pool.close()
+
+
+app = FastAPI(title="Smart Support", version="0.1.0", lifespan=lifespan)
+
+
+@app.websocket("/ws")
+async def websocket_endpoint(ws: WebSocket) -> None:
+    await ws.accept()
+    graph = app.state.graph
+    session_manager = app.state.session_manager
+    settings = app.state.settings
+    callback_handler = TokenUsageCallbackHandler(model_name=settings.llm_model)
+
+    try:
+        while True:
+            raw_data = await ws.receive_text()
+            await dispatch_message(ws, graph, session_manager, callback_handler, raw_data)
+    except WebSocketDisconnect:
+        logger.info("WebSocket client disconnected")
+
+
+if FRONTEND_DIST.is_dir():
+    app.mount("/", StaticFiles(directory=str(FRONTEND_DIST), html=True), name="frontend")