Improve test infrastructure and organization

- Add test-runner stage to Dockerfile for isolated test execution
- Refactor Makefile test targets: separate unit and integration tests
- Unit tests now run on host (fast, no containers required)
- Integration tests run in containers matching production environment
- Add dedicated 'tests' service to docker-compose.yml
- Update test-integration target to use containerized test runner
- Improve service health checks and wait conditions
- Add test environment variables for consistent testing

This change separates unit tests (fast, local) from integration tests
(full environment, containerized) for better developer experience
and more reliable CI/CD pipelines.

Dockerfile

@@ -38,6 +38,29 @@ RUN npm run build:ts
 # Build the application
 RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o main ./cmd/server
 
+# Test runner stage - includes Go runtime and test dependencies
+# This stage is ONLY used for running tests, never deployed to production
+FROM golang:1.25-alpine AS test-runner
+
+RUN apk --no-cache add ca-certificates curl
+
+WORKDIR /app
+
+# Copy everything from builder (includes compiled binary, source code, and dependencies)
+COPY --from=builder /app ./
+
+# Install kepubify for conversion tests
+RUN wget -O /usr/bin/kepubify https://github.com/pgaskin/kepubify/releases/latest/download/kepubify-linux-64bit \
+    && chmod +x /usr/bin/kepubify
+
+# Set test environment variables
+ENV TEST_MODE=true
+ENV RATE_LIMIT_ENABLED=false
+ENV REQUESTS_PER_MINUTE=1000
+
+# Default command for running tests
+CMD ["go", "test", "./cmd/server/tests", "-v", "-timeout", "5m"]
+
 # Final stage
 FROM alpine:latest