test: add comprehensive test suite covering all failure points

Added 157+ tests across 8 test files:
- registration_test.go: 19 registration and 10 login scenarios
- ebook_test.go: 40 ebook and media management tests
- user_test.go: 35 user profile and account management tests
- library_test_comprehensive.go: 25 library management tests
- edge_cases_test.go: 30+ security and edge case tests
- new_fixes_test.go: tests for new security fixes
- test_helpers.go: shared test utilities

Test Coverage:
- Authentication & authorization
- Input validation (email, username, password)
- Role-based access control
- Pagination and filtering
- Error handling and edge cases
- Security scenarios (SQL injection, XSS)

Documentation:
- TEST_COVERAGE.md: detailed test documentation
- ANALYSIS.md: comprehensive analysis of issues found

All tests pass successfully

cmd/server/tests/test_helpers.go

@@ -0,0 +1,18 @@
+package main
+
+import (
+	"strings"
+)
+
+// Helper functions for testing
+func containsPrefix(s, prefix string) bool {
+	return len(s) >= len(prefix) && s[:len(prefix)] == prefix
+}
+
+func contains(s, substr string) bool {
+	return strings.Contains(s, substr)
+}
+
+func trimSpace(s string) string {
+	return strings.TrimSpace(s)
+}