Enable and test Gotify and Custom Webhook notifications

[Wikid82]

Primary goals:

1. Enable a unified wrapper path for outbound provider dispatch.
2. Make Gotify token handling write-only and non-leaking by contract.
3. Add explicit SSRF/redirect/rebinding protections.
4. Add strict error leakage controls for preview/test paths.
5. Add wrapper transport guardrails and expanded validation tests.

[github-advanced-security]

This pull request sets up GitHub code scanning for this repository. Once the scans have completed and the checks have passed, the analysis results for this pull request branch will appear on this overview. Once you merge this pull request, the 'Security' tab will show more code scanning analysis results (for example, for the default branch). Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results. For more information about GitHub code scanning, check out the documentation.

[github-actions]

✅ Supply Chain Verification Results

✅ PASSED

📦 SBOM Summary

• Components: 1670

🔍 Vulnerability Scan

Severity	Count
🔴 Critical	0
🟠 High	0
🟡 Medium	10
🟢 Low	4
Total	14

📎 Artifacts

• SBOM (CycloneDX JSON) and Grype results available in workflow artifacts

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_{Generated by Supply Chain Verification workflow • View Details}

[codecov]

Codecov Report

❌ Patch coverage is 75.18610% with 200 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
backend/internal/notifications/http_wrapper.go	64.10%	67 Missing and 31 partials ⚠️
backend/internal/services/docker_service.go	63.91%	25 Missing and 10 partials ⚠️
...rnal/api/handlers/notification_provider_handler.go	77.66%	18 Missing and 5 partials ⚠️
backend/internal/services/notification_service.go	86.41%	7 Missing and 4 partials ⚠️
backend/internal/api/handlers/security_handler.go	55.55%	4 Missing and 4 partials ⚠️
backend/internal/api/handlers/user_handler.go	88.00%	5 Missing and 1 partial ⚠️
...ackend/internal/api/handlers/permission_helpers.go	28.57%	4 Missing and 1 partial ⚠️
backend/internal/api/handlers/import_handler.go	0.00%	2 Missing and 2 partials ⚠️
frontend/src/pages/UsersPage.tsx	55.55%	3 Missing and 1 partial ⚠️
frontend/src/api/notifications.ts	91.66%	3 Missing ⚠️
... and 1 more

📢 Thoughts on this report? Let us know!