docs: add OpenAPI documentation with Scalar UI

- Add swaggo annotations to all HTTP handlers
- Add Swagger/OpenAPI spec generation with swag
- Create separate docs server binary (drexa-docs)
- Add Makefile with build, run, and docs targets
- Configure Scalar as the API documentation UI

Run 'make docs' to regenerate, 'make run-docs' to serve.

apps/backend/go.mod

@@ -7,6 +7,7 @@ require (
 	github.com/gofiber/fiber/v2 v2.52.9
 	github.com/google/uuid v1.6.0
 	github.com/sqids/sqids-go v0.4.1
+	github.com/swaggo/swag v1.16.6
 	github.com/uptrace/bun v1.2.16
 	github.com/uptrace/bun/extra/bundebug v1.2.16
 	golang.org/x/crypto v0.45.0
@@ -14,9 +15,24 @@ require (
 )
 
 require (
+	github.com/KyleBanks/depth v1.2.1 // indirect
+	github.com/PuerkitoBio/purell v1.1.1 // indirect
+	github.com/PuerkitoBio/urlesc v0.0.0-20170810143723-de5bf2ad4578 // indirect
 	github.com/fatih/color v1.18.0 // indirect
+	github.com/go-openapi/jsonpointer v0.19.5 // indirect
+	github.com/go-openapi/jsonreference v0.19.6 // indirect
+	github.com/go-openapi/spec v0.20.4 // indirect
+	github.com/go-openapi/swag v0.19.15 // indirect
+	github.com/josharian/intern v1.0.0 // indirect
+	github.com/mailru/easyjson v0.7.6 // indirect
 	go.opentelemetry.io/otel v1.38.0 // indirect
 	go.opentelemetry.io/otel/trace v1.38.0 // indirect
+	golang.org/x/mod v0.29.0 // indirect
+	golang.org/x/net v0.47.0 // indirect
+	golang.org/x/sync v0.18.0 // indirect
+	golang.org/x/text v0.31.0 // indirect
+	golang.org/x/tools v0.38.0 // indirect
+	gopkg.in/yaml.v2 v2.4.0 // indirect
 	mellium.im/sasl v0.3.2 // indirect
 )