LINUS ROUND 1: Delete dead code (rust/c), generic Store[T], tests, slim main.go

- Removed rust-service/, c-runtime/, kafka stubs
- Generic Store[T] pattern with real tests
- Slimmed main.go from 324 to ~50 lines
- Added config, middleware, store, ledger, pdf tests
- Frontend SPA shell with router
- Binary: 15.5MB -> 12MB

backend/config/config_test.go

@@ -0,0 +1,74 @@
+package config
+
+import (
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestLoad_Defaults(t *testing.T) {
+	// Clear env vars
+	os.Unsetenv("PORT")
+	os.Unsetenv("DB_URL")
+	os.Unsetenv("JWT_SECRET")
+
+	// JWT_SECRET is required, so this should panic
+	assert.Panics(t, func() {
+		Load()
+	})
+}
+
+func TestLoad_WithEnv(t *testing.T) {
+	os.Setenv("JWT_SECRET", "test-secret")
+	os.Setenv("PORT", "8080")
+	os.Setenv("DB_URL", "postgres://test")
+	defer func() {
+		os.Unsetenv("JWT_SECRET")
+		os.Unsetenv("PORT")
+		os.Unsetenv("DB_URL")
+	}()
+
+	cfg := Load()
+	assert.Equal(t, "8080", cfg.Port)
+	assert.Equal(t, "postgres://test", cfg.DBURL)
+	assert.Equal(t, "test-secret", cfg.JWTSecret)
+}
+
+func TestLoad_CORSOrigins(t *testing.T) {
+	os.Setenv("JWT_SECRET", "test-secret")
+	os.Setenv("CORS_ORIGINS", "http://localhost:3000, http://localhost:3001")
+	defer func() {
+		os.Unsetenv("JWT_SECRET")
+		os.Unsetenv("CORS_ORIGINS")
+	}()
+
+	cfg := Load()
+	assert.Equal(t, []string{"http://localhost:3000", "http://localhost:3001"}, cfg.CORSOrigins)
+}
+
+func TestLoad_KafkaBrokers(t *testing.T) {
+	os.Setenv("JWT_SECRET", "test-secret")
+	os.Setenv("KAFKA_BROKERS", "kafka1:9092,kafka2:9092")
+	defer func() {
+		os.Unsetenv("JWT_SECRET")
+		os.Unsetenv("KAFKA_BROKERS")
+	}()
+
+	cfg := Load()
+	assert.Equal(t, []string{"kafka1:9092", "kafka2:9092"}, cfg.KafkaBrokers)
+}
+
+func TestRequireEnv(t *testing.T) {
+	os.Setenv("TEST_VAR", "test-value")
+	defer os.Unsetenv("TEST_VAR")
+
+	assert.Equal(t, "test-value", requireEnv("TEST_VAR"))
+}
+
+func TestRequireEnv_Missing(t *testing.T) {
+	os.Unsetenv("MISSING_VAR")
+	assert.Panics(t, func() {
+		requireEnv("MISSING_VAR")
+	})
+}