fix(backend): give the rate limiters account-unique namespace IDs

[gcharang]

What

apps/backend/wrangler.jsonc moves RATE_LIMITER from namespace 1001 to 52001 (production) and from 1002 to 52002 (staging).

Why

Rate-limit namespace_ids are account-scoped. An inventory of every Worker's rate-limit bindings in the Cloudflare account (2026-10-09) found:

• 1001: understudy-backend:RATE_LIMITER (300/60), mailflare:LOGIN_RATE_LIMIT (20/60) and wang-fei-voice-studio:VOICE_START_LIMITER (10/60)
• 1002: understudy-backend-staging:RATE_LIMITER (300/60) and mailflare-nomixhq:LOGIN_RATE_LIMIT (20/60)

Workers that share a namespace share its counter space while declaring different limits. Keys don't overlap today, since understudy hashes its keys with telemetryPseudonym and mailflare uses scope:ip, so the conflict is latent. No other binding in the account uses 52001 or 52002. mailflare moves to 51001/51002 separately.

Effect

• The limiter counters reset once when each environment deploys. Limits and periods are unchanged.
• Staging picks this up through the normal deploy workflow on merge.
• Production picks it up on its next deploy; production deploys stay disabled until the protocol-3 cutover.

Verification

• Config-only change. wrangler.jsonc still parses, and the diff touches only the two namespace_id values.
• Not run locally in this repo: no test reads the namespace IDs (rg for 1001/1002/namespace_id found only this file and an unrelated doc).

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