feat(logging): integrate slog logging with env control and request/sync tracing

- Configure structured slog logger in cmd/server/main.go via LOG_LEVEL and DEBUG env vars
- Add HTTP API request logging middleware in internal/api/ with status and duration_ms
- Add WebSocket join/leave, event reception, and debounced SQLite flush tracing in docroom
- Document LOG_LEVEL and DEBUG settings in .env.example, compose.yaml, and docker-compose.yml

internal/ws/docroom.go

@@ -2,7 +2,7 @@ package ws
 
 import (
 	"encoding/json"
-	"log"
+	"log/slog"
 	"net/http"
 	"sync"
 	"time"
@@ -477,13 +477,30 @@ func (r *DocRoom) Flush() {
 	r.dirty = false
 	r.mu.Unlock()
 
+	tagsCount := 0
+	for _, b := range blocksCopy {
+		tagsCount += len(domain.ExtractTags(b.Content))
+	}
+
+	start := time.Now()
 	r.hub.mu.RLock()
 	store := r.hub.docStore
 	r.hub.mu.RUnlock()
 
 	if store != nil {
 		if err := store.FlushDocumentState(docID, title, blocksCopy); err != nil {
-			log.Printf("[doc-room] flush error for doc %s: %v", docID, err)
+			slog.Error("SQLite document flush failed",
+				"doc_id", docID,
+				"error", err,
+			)
+		} else {
+			durationMs := float64(time.Since(start).Microseconds()) / 1000.0
+			slog.Debug("SQLite document flushed",
+				"doc_id", docID,
+				"blocks_count", len(blocksCopy),
+				"tags_count", tagsCount,
+				"duration_ms", durationMs,
+			)
 		}
 	}
 }
@@ -492,7 +509,7 @@ func (r *DocRoom) Flush() {
 func (r *DocRoom) ServeDocWS(w http.ResponseWriter, req *http.Request, userID string, clientID string, canWrite bool) {
 	conn, err := upgrader.Upgrade(w, req, nil)
 	if err != nil {
-		log.Printf("[doc-ws] upgrade error for doc %s: %v", r.docID, err)
+		slog.Error("WebSocket upgrade error", "doc_id", r.docID, "error", err)
 		return
 	}
 
@@ -518,6 +535,7 @@ func (r *DocRoom) ServeDocWS(w http.ResponseWriter, req *http.Request, userID st
 
 	r.mu.Lock()
 	r.wsClients[client] = true
+	activeClients := len(r.wsClients)
 	if r.activeCursors == nil {
 		r.activeCursors = make(map[string]*CursorPresence)
 	}
@@ -542,6 +560,13 @@ func (r *DocRoom) ServeDocWS(w http.ResponseWriter, req *http.Request, userID st
 	}
 	r.mu.Unlock()
 
+	slog.Debug("WS client joined doc room",
+		"doc_id", r.docID,
+		"client_id", clientID,
+		"user_id", userID,
+		"active_clients", activeClients,
+	)
+
 	// Immediately send live in-memory state and current cursors to the newly connected client
 	initEvt := DocSyncEvent{
 		Type:          "doc_state",
@@ -609,6 +634,13 @@ func (c *DocWSClient) readPump() {
 		}
 		c.room.mu.Unlock()
 
+		slog.Debug("WS client left doc room",
+			"doc_id", c.room.docID,
+			"client_id", c.clientID,
+			"user_id", c.userID,
+			"active_clients", remaining,
+		)
+
 		_ = c.conn.Close()
 
 		// Flush immediately when the last client leaves the document room
@@ -632,6 +664,18 @@ func (c *DocWSClient) readPump() {
 
 		var evt DocSyncEvent
 		if err := json.Unmarshal(message, &evt); err == nil && evt.Type != "" {
+			targetBlockID := evt.BlockID
+			if targetBlockID == "" && evt.Block != nil {
+				targetBlockID = evt.Block.ID
+			}
+
+			slog.Debug("WS doc event received",
+				"type", evt.Type,
+				"doc_id", c.room.docID,
+				"block_id", targetBlockID,
+				"sender_client_id", c.clientID,
+			)
+
 			// Read-only clients can still move their cursor and send presence updates
 			if !c.canWrite && evt.Type != "cursor_moved" {
 				continue

internal/ws/hub.go

@@ -3,7 +3,7 @@ package ws
 import (
 	"encoding/json"
 	"fmt"
-	"log"
+	"log/slog"
 	"net/http"
 	"strings"
 	"sync"
@@ -627,7 +627,7 @@ func (c *Client) SendEvent(event string, payload interface{}) {
 func (h *Hub) ServeWS(w http.ResponseWriter, r *http.Request, roomID, userID string) {
 	conn, err := upgrader.Upgrade(w, r, nil)
 	if err != nil {
-		log.Printf("[ws] upgrade error: %v", err)
+		slog.Error("WebSocket upgrade error", "room_id", roomID, "user_id", userID, "error", err)
 		return
 	}