test: add comprehensive Go test suite with ~75% coverage

New test files:
- config/config_test.go (100% coverage)
- constants/constants_test.go (100% coverage)
- httputil/response_test.go (100% coverage)
- validation/rules_test.go (91.9% coverage)
- middleware/logger_test.go, security_test.go, security_logger_test.go
- handlers/errors_test.go

Updated documentation:
- doc/27-GO-TESTING.md: Complete testing guide
- doc/00-GO-DOCUMENTATION-INDEX.md: Added testing section
- doc/01-ARCHITECTURE.md: Updated package structure
- doc/DECISIONS.md: Added ADR-004 caching decision
- PROJECT-MEMORY.md: Added Go testing section

doc/00-GO-DOCUMENTATION-INDEX.md

@@ -29,6 +29,13 @@ This documentation covers the core Go systems that power the CV site, with a foc
    - Protected endpoints and authentication
    - API request/response formats
 
+4. **[Go Testing](27-GO-TESTING.md)** (~450 lines)
+   - Coverage summary by package (100% for config, constants, httputil)
+   - Test file descriptions and locations
+   - Testing patterns (table-driven, HTTP handlers, middleware)
+   - Coverage gap explanations
+   - Best practices and CI/CD integration
+
 ## Quick Navigation
 
 ### By Feature
@@ -54,6 +61,14 @@ This documentation covers the core Go systems that power the CV site, with a foc
 - [PDF Export](26-GO-ROUTES-API.md#exportpdf---pdf-export)
 - [Security Features](26-GO-ROUTES-API.md#security-features)
 
+**Testing:**
+- [Coverage Summary](27-GO-TESTING.md#coverage-summary)
+- [Test Files](27-GO-TESTING.md#test-files)
+- [Running Tests](27-GO-TESTING.md#running-tests)
+- [Test Patterns](27-GO-TESTING.md#test-patterns)
+- [Coverage Gaps](27-GO-TESTING.md#coverage-gaps)
+- [Best Practices](27-GO-TESTING.md#best-practices)
+
 ### By Use Case
 
 **Setting Up Validation:**
@@ -71,6 +86,11 @@ This documentation covers the core Go systems that power the CV site, with a foc
 2. [Register handlers](26-GO-ROUTES-API.md#route-table)
 3. [Apply security](26-GO-ROUTES-API.md#route-specific-middleware)
 
+**Writing Tests:**
+1. [Review existing coverage](27-GO-TESTING.md#coverage-summary)
+2. [Follow test patterns](27-GO-TESTING.md#test-patterns)
+3. [Run and verify](27-GO-TESTING.md#running-tests)
+
 ## System Architecture
 
 ### Overall Flow
@@ -367,6 +387,7 @@ cv/
 │   ├── 24-GO-VALIDATION-SYSTEM.md   # Validation docs
 │   ├── 25-GO-TEMPLATE-SYSTEM.md     # Template docs
 │   ├── 26-GO-ROUTES-API.md          # Routes/API docs
+│   ├── 27-GO-TESTING.md             # Testing & coverage
 │   └── 00-GO-DOCUMENTATION-INDEX.md # This file
 │
 ├── internal/
@@ -473,5 +494,5 @@ This documentation is part of the CV site project.
 ---
 
 **Last Updated:** December 6, 2025
-**Total Documentation:** 2,836+ lines across 3 files
-**Coverage:** Validation, Templates, Routes, Middleware, Security
+**Total Documentation:** 3,300+ lines across 4 files
+**Coverage:** Validation, Templates, Routes, Middleware, Security, Testing