feat: add provider-aware notification setup

[crypt0rr]

Summary

Notification setup currently requires a complete Shoutrrr URL. Add SMTP, Discord webhook, and ntfy forms to the unit and platform consoles so administrators can enter familiar provider details, while retaining Advanced Shoutrrr URL for other providers.

Both destination APIs accept an additive structured config input. The backend validates it with the pinned Shoutrrr parser and writes through the existing encrypted, write-only destination operations. Existing URL clients, password confirmation, revisions, routing, and queued-alert behavior remain compatible. Credential edits replace the complete configuration; test-send feedback asks the recipient to confirm arrival.

Update the notification, first-scan, security, and API compatibility guides for v0.31.0. No database migration or runtime configuration change is required.

Validation

• Full go test -race -timeout=25m ./... and go vet ./... passed.
• Frontend lint/build and coverage passed: 507 tests; release-range frontend diff coverage is 98.26%.
• Playwright passed: 137 tests passed, 211 skipped by project configuration.
• Documentation build and local link checks passed; documentation deployment will be verified after merge.
• Pinned release-range Go lint, govulncheck, module integrity, npm high-severity audit gates, scanner pin, schema docs, and release artifact/workflow contract tests passed.

The docs dependency audit retains its documented 10 moderate build-tool findings; govulncheck reports no reachable vulnerabilities. Required CI coverage and container jobs must pass before merging and tagging.

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