fix(api): a mixed-case grace_period_days key merges instead of replacing #1658

PUT /api/config checks whether the body carries grace_period_days with an exact-case map lookup (internal/api/handler_config.go:75, present["grace_period_days"]). json.Unmarshal matches keys case-insensitively.

Failure scenario: a stored config has {"aws": 7, "azure": 3}, and a client sends {"Grace_Period_Days": {"aws": 5}}. The presence check misses the key, so the stored map is not reset (line 111). json.Unmarshal then merges into it, and the result is {"aws": 5, "azure": 3} instead of {"aws": 5}. The key the client dropped stays.

Impact is low: the frontend sends the exact key. Fix: detect the key with a case-insensitive check, like the one riExchangeModeSent uses (or the bodyHasKey helper from #1656 once it merges), and add a test with a mixed-case key.

Found by the cold review of #1656.