Add logging service

New Go program (logger/) that:
- Accepts POSTed JSON log messages via POST /log
- Stores last N messages in a ring buffer (default 1M)
- Retrieves logs via GET /logs with limit/before/after filters
- Shows status via GET /status

Also updates express/logging.ts to POST messages to the logger service.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

logger/store.go

@@ -0,0 +1,126 @@
+package main
+
+import (
+	"sync"
+)
+
+// Message represents a log entry from the express backend
+type Message struct {
+	Timestamp int64    `json:"timestamp"`
+	Source    string   `json:"source"` // "logging" | "diagnostic" | "user"
+	Text      []string `json:"text"`
+}
+
+// LogStore is a thread-safe ring buffer for log messages
+type LogStore struct {
+	mu       sync.RWMutex
+	messages []Message
+	head     int  // next write position
+	full     bool // whether buffer has wrapped
+	capacity int
+}
+
+// NewLogStore creates a new log store with the given capacity
+func NewLogStore(capacity int) *LogStore {
+	return &LogStore{
+		messages: make([]Message, capacity),
+		capacity: capacity,
+	}
+}
+
+// Add inserts a new message into the store
+func (s *LogStore) Add(msg Message) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	s.messages[s.head] = msg
+	s.head++
+	if s.head >= s.capacity {
+		s.head = 0
+		s.full = true
+	}
+}
+
+// Count returns the number of messages in the store
+func (s *LogStore) Count() int {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+
+	if s.full {
+		return s.capacity
+	}
+	return s.head
+}
+
+// GetRecent returns the most recent n messages, newest first
+func (s *LogStore) GetRecent(n int) []Message {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+
+	count := s.Count()
+	if n > count {
+		n = count
+	}
+	if n == 0 {
+		return nil
+	}
+
+	result := make([]Message, n)
+	pos := s.head - 1
+	for i := 0; i < n; i++ {
+		if pos < 0 {
+			pos = s.capacity - 1
+		}
+		result[i] = s.messages[pos]
+		pos--
+	}
+	return result
+}
+
+// Filter parameters for retrieving logs
+type FilterParams struct {
+	Limit  int   // max messages to return (0 = default 100)
+	Before int64 // only messages before this timestamp
+	After  int64 // only messages after this timestamp
+}
+
+// GetFiltered returns messages matching the filter criteria
+func (s *LogStore) GetFiltered(params FilterParams) []Message {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+
+	limit := params.Limit
+	if limit <= 0 {
+		limit = 100
+	}
+
+	count := s.Count()
+	if count == 0 {
+		return nil
+	}
+
+	result := make([]Message, 0, limit)
+	pos := s.head - 1
+
+	for i := 0; i < count && len(result) < limit; i++ {
+		if pos < 0 {
+			pos = s.capacity - 1
+		}
+		msg := s.messages[pos]
+
+		// Apply filters
+		if params.Before > 0 && msg.Timestamp >= params.Before {
+			pos--
+			continue
+		}
+		if params.After > 0 && msg.Timestamp <= params.After {
+			pos--
+			continue
+		}
+
+		result = append(result, msg)
+		pos--
+	}
+
+	return result
+}