docs: add architecture decision records

docs/adr/0002-no-async-task-queue.md

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+# ADR-0002: Do Not Add an Async Task Queue Yet
+
+Date: 2026-04-28
+
+## Status
+
+Accepted
+
+## Context
+
+The initial booking flow requires email delivery and QR code generation. These operations are important, but the project does not yet require high-volume background processing, scheduled jobs, retries across many task types, or distributed workers.
+
+Adding Celery, Redis, or another queue would increase deployment and operational complexity for a small theatre company website.
+
+## Decision
+
+Do not use Celery, Redis, or any asynchronous task queue at this stage.
+
+The backend will perform initial email sending and QR code generation synchronously within the Django application, with clear error handling and logging that avoids sensitive data.
+
+## Consequences
+
+- The deployment remains small: Django, PostgreSQL, Angular static assets, and nginx.
+- Local development and production operations are easier to understand.
+- Email provider latency or outages can affect booking and confirmation responses.
+- If email volume, retry needs, or long-running work become a real problem, the project can introduce a task queue later with a new ADR.