feat(server): add admin endpoint to delete 7-day inactive hosts

[DrDead0]

Summary

This PR addresses the issue of old, disconnected host records persisting in the database by introducing a secure deletion endpoint.

Scope: Backend only (UI confirmation modal will be implemented in a separate frontend PR).

We added a DELETE /hosts/:id endpoint that strictly enforces:

1. The host exists (returns 404 otherwise).
2. The host is not ONLINE (returns 409 Conflict otherwise).
3. The host has been inactive for at least 7 days using lastSeen (or createdAt as a fallback for legacy records).
   It is protected by the @Roles('ADMIN') guard.

Type of change

• Bug fix
• New feature
• Refactor / cleanup
• Docs
• Infrastructure / CI
• Breaking change

Areas touched

• apps/server (NestJS)
• apps/web (React / Vite)
• apps/agent (Go)
• packages/protocol (shared WebSocket contracts)
• infrastructure / .github/workflows
• docs

Related issues

Closes #125

How to test

1. Ensure the backend is running with a local postgres database (npm run db:migrate).
2. Run npm run test --workspace=@docksight/server and verify that all host deletion tests pass.
3. Start the server (npm run dev:server), open the Swagger UI at http://localhost:3000/api/docs, and verify the DELETE /hosts/:id endpoint is documented with the correct responses and requires an Admin Bearer token.

Checklist

• Change is focused and stays within the existing modular monolith boundaries.
• Ran the relevant checks locally (see below).
• Updated docs under docs/ if module boundaries or the protocol changed.
• Added an ADR under docs/decisions/ if this is a larger architectural change.

Local checks

# packages/protocol must be built first — apps/server and apps/web
# import it from its dist output.
npm run build --workspace=@docksight/protocol
npm run test  --workspace=@docksight/protocol

npm run build --workspace=@docksight/server
npm run lint  --workspace=@docksight/server
npm run test  --workspace=@docksight/server

npm run build --workspace=@docksight/web
npm run lint  --workspace=@docksight/web

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[DrDead0]

I have Added the feature according to the issue with no frontend changes

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