Observability: Prometheus metrics, log panel, runbook

- GET /api/metrics renders Prometheus text (equity, P&L, per-strategy
  equity/pnl/allocation/halted, info) with no client library.
- In-memory ring log handler (app/logs.py) exposed at GET /api/logs and a
  dashboard "Server logs" panel; Runtime mirrors bot events into the logger.
- docs/runbook.md covers deploys, the kill switch, resetting paper state and
  going live. Adds tests for metrics rendering and the log ring.

backend/app/main.py

@@ -10,6 +10,7 @@ from fastapi.middleware.cors import CORSMiddleware
 from app.api.routes import router
 from app.config import Settings, get_settings
 from app.db import init_db
+from app.logs import install_log_capture
 from app.runtime import Runtime
 
 
@@ -22,6 +23,7 @@ def create_app(
 
     @asynccontextmanager
     async def lifespan(app: FastAPI) -> AsyncIterator[None]:
+        install_log_capture()
         init_db()
         app.state.runtime = runtime
         yield