refactor(logging): replace all print/log with structured slog logger

- main.go: 30+ log.Printf → logger.Infof
- plugin/registry: 12 log.Printf → logger.Infof
- db/db.go, ws/hub.go: log → logger
- platform-bridge: fmt.Printf/println → logger.Info
- Logs now: time=... level=INFO msg=...
- 16 packages pass, vet clean

backend/internal/db/db.go

@@ -5,7 +5,7 @@ import (
 	"context"
 	"database/sql"
 	"fmt"
-	"log"
+	"git.yeij.top/AskaEth/Cyrene_v2/internal/logger"
 	"strings"
 	"time"
 
@@ -68,7 +68,7 @@ func Connect(cfg Config) (*sql.DB, error) {
 		return nil, fmt.Errorf("ping db: %w", err)
 	}
 
-	log.Printf("[db] connected to %s:%d/%s (max_open=%d, max_idle=%d)",
+	logger.Infof("[db] connected to %s:%d/%s (max_open=%d, max_idle=%d)",
 		cfg.Host, cfg.Port, cfg.DBName, cfg.MaxOpenConns, cfg.MaxIdleConns)
 
 	return db, nil
@@ -119,14 +119,14 @@ func Transaction(ctx context.Context, db *sql.DB, fn func(tx *sql.Tx) error) err
 
 // RunMigration executes a list of SQL migration statements in order.
 func RunMigration(ctx context.Context, db *sql.DB, name string, statements []string) error {
-	log.Printf("[db] running migration: %s (%d statements)", name, len(statements))
+	logger.Infof("[db] running migration: %s (%d statements)", name, len(statements))
 
 	for i, stmt := range statements {
 		if _, err := db.ExecContext(ctx, stmt); err != nil {
 			return fmt.Errorf("migration %s[%d]: %w", name, i, err)
 		}
 	}
-	log.Printf("[db] migration %s completed", name)
+	logger.Infof("[db] migration %s completed", name)
 	return nil
 }