feat(support): add private report conversations

[veryCrunchy]

Summary

• store per-report maintainer and reporter messages encrypted at rest
• let maintainers ask for details from the private queue and atomically mark the report as needing information
• let reporters read and reply through their existing capability URL
• expose the conversation in the OpenAPI contract and both private portal views

Context

Follow-up to #1.

This provides the service contract needed for Nextcloud Native to show support status changes and maintainer questions inside the app. Capability access remains scoped to the existing private report link; maintainer writes require the authenticated admin session and CSRF token. Message bodies are bounded to 8 KiB and encrypted with report/message-specific associated data.

Validation

On the dedicated build host at commit 76c6f78db32baa0062df4fa85bc3150d44d15920:

• Node 24: npm ci, npm run lint, npm test, npm run build (7 tests passed)
• Go 1.24: go test ./..., go vet ./...
• production multi-stage container build

The migration uses an ON DELETE CASCADE report relationship; in-memory retention tests and storage behavior remove message data with the report.