fix(s3): default S3_REGION to us-east-1 instead of empty string (#495)

The AWS SDK throws "Region is missing" when S3Client is constructed
with an empty-string region, so any S3 feature (avatars, attachments,
presigned URL generation) is silently broken when S3_REGION is unset
— even though .env.example ships it unset and env.ts declares it
optional. The schema and the runtime disagreed about whether the var
was required.

Default to "us-east-1" in createS3Client. S3-compatible providers
(MinIO, Backblaze B2, R2, DigitalOcean Spaces, Wasabi) ignore the
region entirely; real AWS S3 users should set S3_REGION explicitly
to their bucket's actual region (clarified in .env.example).

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/shared/src/utils/s3.ts

@@ -16,8 +16,14 @@ export function createS3Client() {
         }
       : undefined;
 
+  // The AWS SDK throws "Region is missing" if region is undefined or
+  // empty string at S3Client construction. Default to "us-east-1" so
+  // S3-compatible providers (MinIO, Backblaze B2, R2, Spaces, Wasabi)
+  // that don't care about region work without forcing operators to
+  // set a meaningless value. Real AWS S3 users should still set
+  // S3_REGION explicitly to their bucket's actual region.
   return new S3Client({
-    region: process.env.S3_REGION ?? "",
+    region: process.env.S3_REGION ?? "us-east-1",
     endpoint: process.env.S3_ENDPOINT ?? "",
     forcePathStyle: process.env.S3_FORCE_PATH_STYLE === "true",
     credentials,