Add usage limits & quotas guide (plus input validation patterns)

[developerjamiu]

Demand

Kapa "Implementing Free Account Limits" cluster — 7 conversations / 3 users.

Problem

Scopes/authorization work for tiers, but there is no guide on per-user usage quotas, and:

• No count-query vs counter-table trade-off guidance, or how to return a quota-exceeded error.
• No rate-limiting middleware example (the middleware page mentions rate limiting but shows none).
• Input validation patterns are not documented (response typing is).

Scope

• A "Usage limits & quotas" guide: scopes for tiers + counter pattern + quota-exceeded exception + a rate-limit middleware example in 01-working-with-endpoints/04-middleware.md.
• Input validation patterns (link to 04-exceptions.md).

Related

The endpoint-architecture half of this cluster is covered by #630.