fix: prevent filer.backup stall in single-filer setups (#7695)

* fix: prevent filer.backup stall in single-filer setups (#4977)

When MetaAggregator.MetaLogBuffer is empty (which happens in single-filer
setups with no peers), ReadFromBuffer was returning nil error, causing
LoopProcessLogData to enter an infinite wait loop on ListenersCond.

This fix returns ResumeFromDiskError instead, allowing SubscribeMetadata
to loop back and read from persisted logs on disk. This ensures filer.backup
continues processing events even when the in-memory aggregator buffer is empty.

Fixes #4977

* test: add integration tests for metadata subscription

Add integration tests for metadata subscription functionality:

- TestMetadataSubscribeBasic: Tests basic subscription and event receiving
- TestMetadataSubscribeSingleFilerNoStall: Regression test for #4977,
  verifies subscription doesn't stall under high load in single-filer setups
- TestMetadataSubscribeResumeFromDisk: Tests resuming subscription from disk

Related to #4977

* ci: add GitHub Actions workflow for metadata subscribe tests

Add CI workflow that runs on:
- Push/PR to master affecting filer, log_buffer, or metadata subscribe code
- Runs the integration tests for metadata subscription
- Uploads logs on failure for debugging

Related to #4977

* fix: use multipart form-data for file uploads in integration tests

The filer expects multipart/form-data for file uploads, not raw POST body.
This fixes the 'Content-Type isn't multipart/form-data' error.

* test: use -peers=none for faster master startup

* test: add -peers=none to remaining master startup in ec tests

* fix: use filer HTTP port 8888, WithFilerClient adds 10000 for gRPC

WithFilerClient calls ToGrpcAddress() which adds 10000 to the port.
Passing 18888 resulted in connecting to 28888. Use 8888 instead.

* test: add concurrent writes and million updates tests

- TestMetadataSubscribeConcurrentWrites: 50 goroutines writing 20 files each
- TestMetadataSubscribeMillionUpdates: 1 million metadata entries via gRPC
  (metadata only, no actual file content for speed)

* fix: address PR review comments

- Handle os.MkdirAll errors explicitly instead of ignoring
- Handle log file creation errors with proper error messages
- Replace silent event dropping with 100ms timeout and warning log

* Update metadata_subscribe_integration_test.go

.github/workflows/metadata-subscribe-tests.yml

@@ -0,0 +1,92 @@
+name: "Metadata Subscribe Integration Tests"
+
+on:
+  push:
+    branches: [ master ]
+    paths:
+      - 'weed/filer/**'
+      - 'weed/pb/filer_pb/**'
+      - 'weed/util/log_buffer/**'
+      - 'weed/server/filer_grpc_server_sub_meta.go'
+      - 'weed/command/filer_backup.go'
+      - 'test/metadata_subscribe/**'
+      - '.github/workflows/metadata-subscribe-tests.yml'
+  pull_request:
+    branches: [ master ]
+    paths:
+      - 'weed/filer/**'
+      - 'weed/pb/filer_pb/**'
+      - 'weed/util/log_buffer/**'
+      - 'weed/server/filer_grpc_server_sub_meta.go'
+      - 'weed/command/filer_backup.go'
+      - 'test/metadata_subscribe/**'
+      - '.github/workflows/metadata-subscribe-tests.yml'
+
+concurrency:
+  group: ${{ github.head_ref }}/metadata-subscribe-tests
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+env:
+  GO_VERSION: '1.24'
+  TEST_TIMEOUT: '10m'
+
+jobs:
+  metadata-subscribe-integration:
+    name: Metadata Subscribe Integration Tests
+    runs-on: ubuntu-22.04
+    timeout-minutes: 20
+    
+    steps:
+    - name: Checkout code
+      uses: actions/checkout@v6
+      
+    - name: Set up Go ${{ env.GO_VERSION }}
+      uses: actions/setup-go@v6
+      with:
+        go-version: ${{ env.GO_VERSION }}
+        
+    - name: Build SeaweedFS
+      run: |
+        cd weed
+        go build -o weed .
+        chmod +x weed
+        ./weed version
+        
+    - name: Run Metadata Subscribe Integration Tests
+      run: |
+        cd test/metadata_subscribe
+        
+        echo "Running Metadata Subscribe integration tests..."
+        echo "============================================"
+        
+        # Run tests with verbose output
+        go test -v -timeout=${{ env.TEST_TIMEOUT }} ./...
+        
+        echo "============================================"
+        echo "Metadata Subscribe integration tests completed"
+        
+    - name: Archive logs on failure
+      if: failure()
+      uses: actions/upload-artifact@v5
+      with:
+        name: metadata-subscribe-test-logs
+        path: |
+          /tmp/seaweedfs_*
+        retention-days: 7
+        
+    - name: Test Summary
+      if: always()
+      run: |
+        echo "## Metadata Subscribe Integration Test Summary" >> $GITHUB_STEP_SUMMARY
+        echo "" >> $GITHUB_STEP_SUMMARY
+        echo "### Test Coverage" >> $GITHUB_STEP_SUMMARY
+        echo "- **Basic Subscription**: Subscribe to metadata changes and receive events" >> $GITHUB_STEP_SUMMARY
+        echo "- **Single-Filer No Stall**: Regression test for issue #4977" >> $GITHUB_STEP_SUMMARY
+        echo "- **Resume from Disk**: Verify subscription can resume from persisted logs" >> $GITHUB_STEP_SUMMARY
+        echo "" >> $GITHUB_STEP_SUMMARY
+        echo "### Related Issues" >> $GITHUB_STEP_SUMMARY
+        echo "- [#4977](https://github.com/seaweedfs/seaweedfs/issues/4977): filer.backup synchronisation stall" >> $GITHUB_STEP_SUMMARY
+