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/logs.py

@@ -0,0 +1,55 @@
+from __future__ import annotations
+
+import logging
+from collections import deque
+from datetime import UTC, datetime
+
+
+class RingLogHandler(logging.Handler):
+    """Keeps the most recent log records in memory for the dashboard.
+
+    Deliberately small and dependency-free: the goal is a debugging aid, not a
+    log store. Attach once at startup.
+    """
+
+    def __init__(self, capacity: int = 200) -> None:
+        super().__init__()
+        self.records: deque[dict] = deque(maxlen=capacity)
+
+    def emit(self, record: logging.LogRecord) -> None:
+        try:
+            self.records.append(
+                {
+                    "time": datetime.fromtimestamp(record.created, UTC).isoformat(),
+                    "level": record.levelname.lower(),
+                    "logger": record.name,
+                    "message": record.getMessage(),
+                }
+            )
+        except Exception:  # never let logging break the app
+            self.handleError(record)
+
+    def snapshot(self, limit: int = 100) -> list[dict]:
+        items = list(self.records)
+        return items[-limit:]
+
+
+_handler: RingLogHandler | None = None
+
+
+def install_log_capture(capacity: int = 200) -> RingLogHandler:
+    """Attach the ring handler to the root logger (idempotent)."""
+    global _handler
+    if _handler is not None:
+        return _handler
+    _handler = RingLogHandler(capacity)
+    _handler.setLevel(logging.INFO)
+    root = logging.getLogger()
+    root.addHandler(_handler)
+    if root.level > logging.INFO or root.level == logging.NOTSET:
+        root.setLevel(logging.INFO)
+    return _handler
+
+
+def get_log_handler() -> RingLogHandler | None:
+    return _handler