fix(server): replace MD5 with SHA-256 in FsHashStore

SonarCloud S4790: weak hash algorithm. SHA-256 is content
fingerprint only (not security) but S4790 flags any MD5 use.

server/server/internal/objects/fsBackend.ts

@@ -162,7 +162,7 @@ export class FsObjectBackend extends ObjectBackend {
     if (obj === undefined) return;
 
     // hash object
-    const hash = createHash("md5");
+    const hash = createHash("sha256");
     hash.setEncoding("hex");
 
     // local variable to point to object