Implement node status synchronization in Controller, including downtime notifications and online/offline management

[Paragrf]

Background
To improve cluster reliability and operational efficiency, the controller needs to bridge the status gap between slave nodes and the server. Beyond just reporting unexpected failures, it is essential to support proactive maintenance workflows, allowing operators to safely remove traffic before performing node updates or hardware swaps.

Key Changes

• Downtime Push: The controller now monitors slave node health and proactively pushes "Downtime" alerts to the server upon detection of a crash or heartbeat timeout.

• Manual Offline (Traffic Draining): Supports a proactive "Offline" command. This allows the server to drain/stop traffic to a specific slave node before any maintenance work begins, ensuring zero-impact operations.

• Manual Online: Supports a "Ready-to-Serve" notification when a slave is back online and fully synchronized, allowing the server to safely re-enable traffic.

Related Issues
Fixes #385

[codecov-commenter]

Codecov Report

❌ Patch coverage is 79.04762% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.09%. Comparing base (6c56470) to head (87937e4).
⚠️ Report is 105 commits behind head on unstable.

Files with missing lines	Patch %	Lines
controller/cluster.go	54.16%	7 Missing and 4 partials ⚠️
server/api/node.go	73.91%	4 Missing and 2 partials ⚠️
server/helper/helper.go	0.00%	2 Missing ⚠️
store/cluster.go	95.45%	1 Missing and 1 partial ⚠️
server/route.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #388      +/-   ##
============================================
+ Coverage     43.38%   50.09%   +6.70%     
============================================
  Files            37       45       +8     
  Lines          2971     3885     +914     
============================================
+ Hits           1289     1946     +657     
- Misses         1544     1724     +180     
- Partials        138      215      +77     

Flag	Coverage Δ
unittests	50.09% <79.04%> (+6.70%)	⬆️

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