docs: add architecture documentation and slides

docs/adr/004-sms-provider-strategy.md

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+# ADR 004: Select SMS Providers With Strategy
+
+## Status
+
+Accepted.
+
+## Context
+
+The project supports Kavenegar, SMS.ir, and local debug delivery.
+
+## Decision
+
+Expose a stable `SmsClient` port and choose the concrete provider through a small factory based on `SMS_PROVIDER`.
+
+## Consequences
+
+Adding a new SMS provider does not change the auth use case or worker flow. Tests can mock providers without network access.
+