chore: rename remaining Cloudflare identifiers to piyaz

[FrkAk]

Summary

Task Reference: [MYMR-226]

Renames the remaining mymir Cloudflare identifiers to piyaz — the in-repo part of the cutover. After this, no mymir identifiers remain in code/config.

What ships here:

• Realtime broker / Durable Object rename: DO class MymirBroker → PiyazBroker, binding MYMIR_BROKER → PIYAZ_BROKER, WS headers X-Mymir-User-Id/X-Mymir-Broker-Sig → X-Piyaz-*, DO instance name mymir-broker-global → piyaz-broker-global. (worker-cf.ts, lib/realtime/*, app/api/events/route.ts, components/providers/RealtimeBridge.tsx)
• Auth: env var MYMIR_VERIFIED_OAUTH_CLIENT_IDS → PIYAZ_VERIFIED_OAUTH_CLIENT_IDS (lib/auth/verified-oauth-clients.ts).
• wrangler.jsonc: worker mymir/mymir-production → piyaz/piyaz-production, R2 piyaz-isr-cache, D1 piyaz-tag-cache (new database_id), DO binding/class PIYAZ_BROKER/PiyazBroker, migration v1 new_sqlite_classes. AUTH_KV unchanged (id-bound).
• Deploy guard: placeholder bucket regex mymir-placeholder- → piyaz-placeholder- (scripts/assert-deploy-ready.ts).
• Tests: broker DO + verified-oauth-clients suites updated to the new names.
• cloudflare-env.d.ts: regenerated via cf-typegen (PIYAZ_BROKER/PiyazBroker, dead MYMIR_DB_DRIVER dropped).
• Docs: corrected a stale comment in open-next.config.ts (cited the deleted scripts/postbuild-cf.ts).

Type of change

• Refactor / cleanup
• Documentation

Testing

• Tested locally with bun run dev
• Linting passes (bun run lint)
• Typecheck passes (bun run typecheck)

Notes for reviewer

• This is the in-repo half of the Cloudflare cutover. Provisioning, secrets, deploy:cf, the app.piyaz.ai custom-domain move, and decommissioning the old mymir-* resources are operator steps not in this PR.
• The DO uses a fresh v1 migration because piyaz-production is a new worker (no renamed_classes needed); there is no live DO state to preserve across the worker swap. Do not deploy this onto the existing mymir-production worker — v1 would be treated as already-applied and PIYAZ_BROKER would not bind.
• cloudflare-env.d.ts is the real cf-typegen output. It carries unrelated runtime-type churn from wrangler 4.101.0 (vs the committed ^4.97.0-era output), including a new ambient Agent Memory type section. These are generated, unreferenced by our code, and harmless.