Add resource limits to daemon (bd-152)

Implemented connection limiting, request timeouts, and memory pressure detection:

- Connection limiting with semaphore pattern (default 100 max connections)
- Request timeout enforcement on read/write (default 30s)
- Memory pressure detection with aggressive cache eviction (default 500MB threshold)
- Configurable via environment variables:
  - BEADS_DAEMON_MAX_CONNS
  - BEADS_DAEMON_REQUEST_TIMEOUT
  - BEADS_DAEMON_MEMORY_THRESHOLD_MB
- Health endpoint now exposes active/max connections and memory usage
- Comprehensive test coverage for all limits

This prevents resource exhaustion under heavy load or attack scenarios.

Amp-Thread-ID: https://ampcode.com/threads/T-44d1817a-3709-4f1d-a27a-78bb2fa4d3dc
Co-authored-by: Amp <amp@ampcode.com>

internal/rpc/protocol.go

@@ -150,6 +150,9 @@ type HealthResponse struct {
 	CacheHits       int64   `json:"cache_hits"`
 	CacheMisses     int64   `json:"cache_misses"`
 	DBResponseTime  float64 `json:"db_response_ms"`
+	ActiveConns     int32   `json:"active_connections"`
+	MaxConns        int     `json:"max_connections"`
+	MemoryAllocMB   uint64  `json:"memory_alloc_mb"`
 	Error           string  `json:"error,omitempty"`
 }