Add debug mode for logging server-side API calls

Implements issue #762: Add debug logging for all Hive API calls to help
identify excessive calls and debug production issues.

Features:
- Logs all wax API calls (bridge, condenser_api, database_api, etc.)
- Logs third-party API calls (hiveposh, hivebuzz, peakd)
- Configurable via environment variables
- No performance impact when disabled

Configuration:
- DEBUG_API_CALLS=true     Enable API call logging
- DEBUG_API_LEVEL=all      Log level: all, slow, errors
- DEBUG_API_SLOW_THRESHOLD=1000  Slow call threshold (ms)

Log format (single-line JSON):
{"level":"INFO","api":"bridge.get_post","params":{...},"status":"success","duration_ms":150}

Closes #762