chore(release): update v0.9.81 SHA256 digests after rebuild

Re-cut v0.9.81 binaries from current main (which now includes the
private gate + DM hashchain spool from #326 and the gate-directory
test from #327). All three artifacts were signed with the same
minisign updater key as the original v0.9.81 release, so existing
v0.9.81 installs on Tauri auto-update accept the new bundles.

Updated hashes (verified against released assets):
- ShadowBroker_v0.9.81.zip      f81f454bdc88e9a32c351df38212b8cfa624704d65764b971bb091eef62259c6
- ShadowBroker_0.9.81_x64-setup.exe   25e9a95d0d8ce959a7d08fe8e7406772ae24b596652793e81d1de5d02510a5a6
- ShadowBroker_0.9.81_x64_en-US.msi   34e655fc0c0f195ee4ac978f228a4b2b9d5565253b8771aca9ef4693409e9e70

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/data/release_digests.json

@@ -43,8 +43,8 @@
     "ShadowBroker_0.9.8_x64_en-US.msi": "fe22f9d51e4360d74c18a7250c2fbb9ed4fa4c7a884b3ac0d04a21115466386b"
   },
   "v0.9.81": {
-    "ShadowBroker_v0.9.81.zip": "af8c87ccdece8fbb9aadc6be63cce10d3fcba74e6d87ef83289dda6d555fd270",
-    "ShadowBroker_0.9.81_x64-setup.exe": "4e866fa0423c0c2470ed32f4809167a7815dc23ee7762b69e95681c1f3a28250",
-    "ShadowBroker_0.9.81_x64_en-US.msi": "8977c9a1c54e1f0d030436be9c4e3d81d766cc0080699eb747649095f360c7ff"
+    "ShadowBroker_v0.9.81.zip": "f81f454bdc88e9a32c351df38212b8cfa624704d65764b971bb091eef62259c6",
+    "ShadowBroker_0.9.81_x64-setup.exe": "25e9a95d0d8ce959a7d08fe8e7406772ae24b596652793e81d1de5d02510a5a6",
+    "ShadowBroker_0.9.81_x64_en-US.msi": "34e655fc0c0f195ee4ac978f228a4b2b9d5565253b8771aca9ef4693409e9e70"
   }
 }