Implement Sprint 2 security enhancements

Add user-based rate limiting middleware (100 req/min) for authenticated endpoints using httprate library. Implement common password validation blocking 50+ weak passwords. Improve test coverage from 38.3% to 44.6% with comprehensive refresh token tests.

Security improvements:
- Rate limiting by user ID for /habits and /stats endpoints
- X-RateLimit-Limit header in responses
- Common password blacklist in password validation
- Refresh token test suite with 5 scenarios (valid, invalid, expired, revoked, empty)

internal/application/commands/unmark_habit_test.go

@@ -137,4 +137,3 @@ func TestUnmarkHabitHandler_ReturnsErrorWhenEntryNotFound(t *testing.T) {
 		t.Errorf("Expected ErrNotFound for missing entry, got %v", err)
 	}
 }
-