Each billing sync (Gaiia, Splynx, VISP, Sonar) already computes
subscriber counts and MRR but only stored them in sync messages.
Now persists them on the integrations table so the dashboard can
aggregate across all billing providers instead of only querying
Gaiia network sites (which were never populated).
Also fixes SnmpKit Config environment detection to use Mix.env()
as fallback when MIX_ENV env var is not set.
- Added last_sync_message field to integrations schema
- Gaiia sync: shows item counts on success, friendly error on failure
- Preseem sync: shows AP/device counts on success, friendly error on failure
- Error messages translated from technical errors to user-friendly text
- Message displayed on integration card next to status badge
- Exposed in REST API response
