[M2] Create Loki Push Handler

[miciav]

Description

Implement non-blocking HTTP push to Loki.

Files to create

• lb_common/handlers/loki_handler.py (new)

Key features

• Async HTTP POST to Loki push API (/loki/api/v1/push)
• Batching with configurable flush interval
• Non-blocking queue with overflow protection
• Automatic retry with exponential backoff
• Graceful fallback to file-only on errors

Loki push format

{
  "streams": [{
    "stream": {
      "component": "runner",
      "host": "target-1",
      "run_id": "run-xxx"
    },
    "values": [
      ["1704547800000000000", "{\"level\":\"INFO\",...}"]
    ]
  }]
}

Acceptance criteria

• Non-blocking push (queue-based)
• Batching for efficiency
• Retry logic with backoff
• Falls back silently on failure
• Unit tests with mocked Loki

Milestone: M2 - Loki Push Integration
Issue: 2.1

[miciav]

Implemented Loki push handler in lb_common/handlers/loki_handler.py.