fix: resolve issues #391, #393, #395 assigned to Mai-Fura

[Mai-Fura]

Summary

Resolves three issues assigned to Mai-Fura:

#391 — Complete ADR documentation

• Updated 0003-polling-based-processing.md with problem statement, considered alternatives (Kafka, NATS, Redis/Bull, RabbitMQ), and explicit revisit conditions
• Created 0008-horizontal-worker-scaling.md documenting the distributed lock manager approach
• Created docs/adr/README.md as a full ADR index

#393 — Stream visibility (public/private)

• Added StreamVisibility type and IsOptionalStreamVisibility validator
• Updated DTOs (create, update, list) with visibility + ownerOnly filter
• Updated both repositories (in-memory + PostgreSQL) with ACL: private streams hidden from non-owners
• DB migration 2026080501_add_stream_visibility with CHECK constraint
• Updated service, controller, and all tests

#395 — SDK README fixes

• Audited README against actual SDK surface
• Removed references to non-existent methods/endpoints
• Added typedoc config, updated package.json and .gitignore

Bonus

• Fixed api/src/global.d.ts to add missing Transform and IsBoolean type declarations

Testing

• All 43 stream-related tests pass
• SDK typechecks pass cleanly