P0: Add clear quota errors

[devcodex2025]

Source: docs/GROWTH_AND_AGENT_BACKLOG.md

Area: User limits / API UX
Priority: P0
Owner type: agent or human contributor
Files likely affected: server.js, src/services/user-service.js, web/app

Goal:
Explain credits, plan limits, and next step when analysis is blocked by quota.

Acceptance criteria:

• Implementation matches the backlog goal.
• Tests are added or a clear reason is documented if tests are not applicable.
• User-facing errors or recommendations are actionable and do not leak secrets.
• Documentation/API notes are updated when behavior changes.

Risk:
Medium. Keep the change scoped and avoid unrelated refactors.