Improve README SEO and increase test coverage to 50%

Optimized README for better discoverability with keywords: api, habits, self-hosted. Consolidated content to reduce redundancy while maintaining clarity.

Added comprehensive test coverage across multiple layers:
- Infrastructure: bcrypt hashing, JWT tokens, configuration validation
- Application commands: user deletion, password reset, token revocation, email verification
- Application queries: login, token refresh
- Domain entities: refresh tokens, password reset tokens

Coverage increased from 42.5% to 50.8% with meaningful business logic tests.

Fixed integration test handler initialization with correct parameters.

internal/shared/errors/errors.go

@@ -3,11 +3,11 @@ package errors
 import "errors"
 
 var (
-	ErrNotFound            = errors.New("resource not found")
-	ErrAlreadyExists       = errors.New("resource already exists")
-	ErrInvalidInput        = errors.New("invalid input")
-	ErrUnauthorized        = errors.New("unauthorized")
-	ErrInvalidCredentials  = errors.New("invalid credentials")
-	ErrEmailNotVerified    = errors.New("email not verified")
-	ErrRegistrationClosed  = errors.New("registration is closed")
+	ErrNotFound           = errors.New("resource not found")
+	ErrAlreadyExists      = errors.New("resource already exists")
+	ErrInvalidInput       = errors.New("invalid input")
+	ErrUnauthorized       = errors.New("unauthorized")
+	ErrInvalidCredentials = errors.New("invalid credentials")
+	ErrEmailNotVerified   = errors.New("email not verified")
+	ErrRegistrationClosed = errors.New("registration is closed")
 )