Add mock server mode using the OpenAPI spec

[Jayy4rl]

Context: Frontend developers need a mock backend returning realistic data, without a running PostgreSQL or RPC.

Requirements:

• Install @stoplight/prism-cli.
• Add "mock-server": "prism mock api/openapi.json --port 3001" to backend/package.json.
• Ensure all OpenAPI response schemas include at least one example.

Acceptance criteria:

• npm run mock-server starts a server at port 3001.
• GET http://localhost:3001/api/v1/vaults returns a valid mocked vault list.