Mempool: fix npmDeps fetcher — postUnpack instead of sourceRoot, postPatch instead of patches

The `patches` attribute was being applied inside the npmDeps
fixed-output derivation where patch runs interactively and fails.
sourceRoot also caused the deps fetcher to hash a different tree than
expected. Now:

- postUnpack cds into the subdir for both the fetcher and main build
- postPatch applies the mining-pool patch only in the main build
- npmDepsHash values remain placeholders (lib.fakeHash)

packages/mempool/default.nix

@@ -80,19 +80,26 @@ in rec {
     pname = "mempool-backend";
     inherit version src;
 
-    sourceRoot = "source/backend";
+    # Use postUnpack (not sourceRoot) so the npmDeps fixed-output
+    # derivation hashes the same unpacked tree as the main build.
+    postUnpack = ''
+      cd source/backend
+    '';
 
     # Placeholder: build once, then replace with the hash from the
     # "hash mismatch" error output.
     npmDepsHash = lib.fakeHash;
 
-    patches = [ ./0001-allow-disabling-mining-pool-fetching.patch ];
-
     nativeBuildInputs = [
       makeWrapper
       rsync
     ];
 
+    # Apply patches only in the main build, not in the npmDeps fetcher.
+    postPatch = ''
+      patch -p1 < ${./0001-allow-disabling-mining-pool-fetching.patch}
+    '';
+
     dontNpmBuild = true;
     dontNpmInstall = true;
 
@@ -141,7 +148,11 @@ in rec {
     pname = "mempool-frontend";
     inherit version src;
 
-    sourceRoot = "source/frontend";
+    # Use postUnpack (not sourceRoot) so the npmDeps fixed-output
+    # derivation hashes the same unpacked tree as the main build.
+    postUnpack = ''
+      cd source/frontend
+    '';
 
     # Placeholder: build once, then replace with the hash from the
     # "hash mismatch" error output.