fix: Make push notifications optional for MIPS compatibility

- Add push-notifications feature flag (enabled by default)
- Make web-push and openssl optional dependencies
- Conditionally compile push notification code
- Disable push notifications for MIPS builds in CI
- Fixes AtomicU64 error on 32-bit MIPS architecture
- x86_64/ARM builds still have full push notification support

.github/workflows/build-mips.yml

@@ -55,7 +55,7 @@ jobs:
         CROSS_NO_WARNINGS: 0
       run: |
         cd backend
-        cross build --target mips-unknown-linux-musl --release -Z build-std=std,panic_abort
+        cross build --target mips-unknown-linux-musl --release --no-default-features -Z build-std=std,panic_abort
 
     - name: Debug - List Files
       run: |