Add 7 ADRs documenting backend architecture decisions

docs/decisions/0003-rest-api-with-swagger.md

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+# ADR 0003: REST API with Swagger Documentation
+
+**Date:** 2026-04-26
+**Status:** Accepted
+
+## Context
+
+The API must be understandable and testable without reading source code. Two documentation approaches were considered:
+- **OpenAPI/Swagger** — industry standard, generates interactive UI, supports code generation.
+- **Postman collection** — already present (`tests/adastra-api-tests.postman_collection.json`), good for integration testing but not a substitute for living documentation.
+
+## Decision
+
+Use `swagger-jsdoc` + `swagger-ui-express` to generate and serve an interactive OpenAPI 3.0 documentation at a dedicated route. Annotations are written as JSDoc comments directly in route/controller files.
+
+Postman collections are kept for integration/regression testing (run via `npm run test:postman` with Newman), complementing rather than replacing Swagger.
+
+## Consequences
+
+- **Positive:** Living documentation — always in sync with the code.
+- **Positive:** Interactive UI allows manual endpoint testing without a separate tool.
+- **Positive:** OpenAPI spec can be used to generate client types if needed.
+- **Negative:** JSDoc annotations add verbosity to route files. Annotations must be kept up to date manually.