feat: add JSON template support for all services and fix uptime monitoring reliability
BREAKING CHANGE: None - fully backward compatible
Changes:
- feat(notifications): extend JSON templates to Discord, Slack, Gotify, and generic
- fix(uptime): resolve race conditions and false positives with failure debouncing
- chore(tests): add comprehensive test coverage (86.2% backend, 87.61% frontend)
- docs: add feature guides and manual test plan
Technical Details:
- Added supportsJSONTemplates() helper for service capability detection
- Renamed sendCustomWebhook → sendJSONPayload for clarity
- Added FailureCount field requiring 2 consecutive failures before marking down
- Implemented WaitGroup synchronization and host-specific mutexes
- Increased TCP timeout to 10s with 2 retry attempts
- Added template security: 5s timeout, 10KB size limit
- All security scans pass (CodeQL, Trivy)
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