refactor(internal): replace log.Fatal with panic, add timezone to DSN, add empty S3 bucket check

Replace log.Fatal in dev service init with panic for consistency. Add timezone=UTC to DAV connection DSN. Add IsEmpty() check for S3 dev bucket.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/internal/dav/service_dev.go

@@ -6,7 +6,6 @@ package dav
 import (
 	"context"
 	"fmt"
-	"log"
 	"os"
 
 	"github.com/jackc/pgx/v5/pgxpool"
@@ -20,13 +19,13 @@ func init() {
 		if os.Getenv("GO_TESTING") != "" {
 			return
 		}
-		log.Fatalf("failed to initialize dev service: %v", err)
+		panic("failed to initialize dev service: " + err.Error())
 	}
 }
 
 func connect() error {
 	dsn := fmt.Sprintf(
-		"postgres://%s:%s@localhost:5432/%s?sslmode=disable&require_auth=scram-sha-256",
+		"postgres://%s:%s@localhost:5432/%s?sslmode=disable&timezone=UTC&require_auth=scram-sha-256",
 		getEnv("POSTGRES_USER"),
 		getEnv("POSTGRES_PASSWORD"),
 		getEnv("POSTGRES_DB"),